Create and encrypt RSA or EC private key using general utilities. There are multiple possibilities, so let me sum it up.

genpkey

The genpkey command can be used to generate a private key.

Generate an unencrypted RSA private key.

$ openssl genpkey -algorithm rsa -pkeyopt rsa_keygen_bits:2048 -out rsa_pkey_unenc_genpkey.pem

..............+++++
..+++++

$ cat rsa_pkey_unenc_genpkey.pem

-----BEGIN PRIVATE KEY-----
MIIEugIBADANBgkqhkiG9w0BAQEFAASCBKQwggSgAgEAAoIBAQDIBkKBy5F6pPIV
/dG37YQZ6NnCLSsk/6vEDS9NyU5g2vkUn+rgQZ1Xn3VRzSdf2hltf4Lld+3dL/Yu
dJtCxKxCYVR2PPYuhBCFrY5Xi48ojcEfavAXhbM4QDhvh5VhZZAjgK7kqcT+NC/j
LvH/PYKjhlwsJ5mRx0yrcHR0O4pV9ZiJwloHJFwHfa6q0LHEWwdu4rxRacC6UKjb
/Xw2JkcoEKMejNctuSEfoVyIVt+c92vMi3BvjoiozmB1FblDTkSmDWPUsxCrywZm
9f4FGFUKVItYB9zBwYgNamIXuJ4r42DjFc4S/27TgvxAqZ+BKC0Mc7KTSTNAluXD
bspvHWRhAgMBAAECggEATKNJXB25EbqdHpvvfH8+fAbhWCqiOzzSfP1oQG9+3lFH
2gUjouTA+k+YTc1C5tDTqUaPyofW1UaJFmeLDVv5fD16dQsKb+wdwHos7ZIFOc6G
ThlVAE4zyrugY+sY6wZU4aVuXSJG3fb02iCki9zvbuQN8MUDa/1mfNFLbTVa3W/b
VzCDdQQ4+s3ALWGoxztklR2Ia+AqW3zPf+Y74BV2X1fZUp5wT64oe1Z2dXGXc89E
YTf5ru9dYbL3IMIyFYnah0hNbouteu8M4XWfQmk6a/1eee3IJhgrgGagWDYD7R0Z
bvzxQkm42Ne6ghvBwLie2heK5+EMP6j/rNjIHm2KuQKBgQD/9pg5vjGpljVsvVCv
686rG4UTtlKzE9jH+wmHHnGb3leJi3tGbpCJEScMdgHnLUUxHaznL+ETHoeD7i/q
Jljc0vPrHyurlczZW/AGUIF1vucNBzaJMVZ4DdcRjMqYlrlY3V639WeRpYiWLObC
20XFr5zCAGJYGblWCiaXDToLSwKBgQDIDZwUsAto/Hqxbj716JLD3MEgjCiLuheb
GI9aogpIgl1wVJMoFQmjznaK6z5EVcXXvpYpbO7t2jQDK1oq9CNNrDTuESGQxqtV
BfDlqOFerkL0XLkU4rsEVEu3fBT8eiL4gSI8HExy+zNQi/WNtlseW7GYT2kRLckT
clG2j6W3gwKBgDcZwpNLmi1DpKSP7HqybH7xIaioqwzZzOTf6aEosJQ5PObjG9qB
mqOnCZzpGmfFmwuDs+DgsbcH72w7Lh1du+ttjgyjXSdGzB5OaFEzq2rJebuYPeky
scaOVCn3APXwHF5A6U74iR8VX13UtKhlw5NWxowMwR7uCrO246/IrSIZAn9c/faP
p0Wmh7aAw76AkJuob/VWK9CtItPWRJRejPQ6wF6u2KGPADh67H4uFSvcmAJfRziG
CR0Hc2a6EmzXiqTedi4ke9IeejsqSpiwKNCaMmbtMCzYiIxmdsi7gfI/aobW/S/s
wTrIN/yidGywcxPoCLeFFWTBl4bgg0N4mU1vAoGAHZqYIsCyXCtCwjtbtLKW89vH
jBxPbiFhw5cohl9rxEvAzYdu0mV5OX5XZjSeu4a9LV3OgRP2lOrbgYLPmFkFq07/
tkjHSGJDoEjdmIYQaDA/Tm8B6VKHx+47r8mlu0etSrbjCdOZ4NMZDI8XTnYVtcOg
DEbz50Z96knGnafgZSU=
-----END PRIVATE KEY-----

Generate encrypted RSA private.

$ openssl genpkey -algorithm rsa -pkeyopt rsa_keygen_bits:2048 -aes-256-cbc -pass pass:keypass -out rsa_pkey_enc_genpkey.pem

$ cat rsa_pkey_enc_genpkey.pem    

-----BEGIN ENCRYPTED PRIVATE KEY-----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-----END ENCRYPTED PRIVATE KEY-----

Generate an unencrypted EC private key.

$ openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:prime256v1 -out ec_pkey_unenc_genpkey.pem

$ cat ec_pkey_unenc_genpkey.pem

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg775+kyqsBI+DAY+s
0yEdgGDcEVhp/iVOMynQQQqD99mhRANCAAQcyW+6R3oXG4QbWU8NwADpW9irvnj6
FnELlDvtTem+ufUhiBWGhwCBwVfyBpqJsYuDWLLtHTRY4EVolbQ3K/YB
-----END PRIVATE KEY-----

Generate encrypted EC private key.

$ openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:prime256v1 -aes-256-cbc -pass pass:keypass -out ec_pkey_enc_genpkey.pem

$ cat ec_pkey_enc_genpkey.pem

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIHsMFcGCSqGSIb3DQEFDTBKMCkGCSqGSIb3DQEFDDAcBAi2zHpFW24wnAICCAAw
DAYIKoZIhvcNAgkFADAdBglghkgBZQMEASoEEEajoQzUQgXpJQkpFpBaAO4EgZB5
XSR1KpbsPQ/d9JgPP20bNYKKvWRwAmQRu68GYif1hdv1d0TFdQo3wYkKnebNJvEB
3De33doXjsXP9yzZwfD9cwuPfIQ2JxIt/LqHUVTL7RhtQLb8MKcU8hKQZdGIzooS
lJpqqJ8TGzMMR8Kt2Wi1ACBHOoZWmJvrnNXPUSSwPiCoFiYBKUn+WGv/1ssYRP4=
-----END ENCRYPTED PRIVATE KEY-----

pkey

The pkey command can be used as a public or private key processing tool.

Encrypt existing RSA private key.

$ openssl pkey -aes256 -in rsa_pkey_unenc_genpkey.pem -passout pass:keyout -out rsa_pkey_enc_pkey.pem

$ cat rsa_pkey_enc_pkey.pem

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIFHTBXBgkqhkiG9w0BBQ0wSjApBgkqhkiG9w0BBQwwHAQIq+wjfjCVZRECAggA
MAwGCCqGSIb3DQIJBQAwHQYJYIZIAWUDBAEqBBDQzQWV4ng2lNy45ZMzwBNoBIIE
wCSyGgOGp7fnzw0fgdY9b1+W7G+tcc+lftypKcvjNitt2KlJmwAz0++uWd+2q1I8
NbWYrndYTjxBTdsjIwb1TdLz3MP1L48Vw0ch08rup2IVdZ7QvMl+1GZLzB0HlJkY
dAMDwgNfebbrJEg5QNQfW0tNWrL+3qfC9qCzFcczOBLUqN7hqYcUxmO8CSnsSP0N
q5O0Lxsls92CguCf23NprxjOfRCbxUfclyxddokXIWChy6O/skmMfpT6gUobWF2K
pszZsNP1PRXHziZve50vVcWcd/Nhzl8N/NNQHE5PD4rjwYpYifxZ9LcrSi3s86rh
WS8gT40J2Y7B3oDNhhZkvqW91R21seJzKnd6yT1EM2akGgX8M2SVvGTbsGZmHIHW
os7C8o5YLjBBpjXd1mWU2dvq+evPOod8Q1mJF+mJvALyDtaIls0lpT3rfta7W1OH
rSrYFVmhJcma09R5VAKskf8OgBTHj6GgjJstP1anrwSRXeG5dig0bfU/rgcaz769
RPftxJEpiSA5dSCW03056oFeayt3P/aDw1C0acLj06gxc+kdOEiKyZb1VnV4VFcJ
V8ESdx2EBWABdc+n4eOTm7G2ZjCzt1wx8+0dO8xnG6PvFjRkdFvKjX4MY9MVPs9s
88QCfuJsdbXYcJ/MeZudN8xEooHL6QJmvB9FxGuZdg36D0bA8Cx11UzZccb1InYi
3/ieK1R0iM15GIMslVhZ+ymUNiabECKG1u5S459rzJbC7F4bkGndYVD6f/eHuUU7
gvtqDyWFIKj1wgSoYAyXwIPGNSN/D8b0QGWWeCSC4g2qkf7eChblJTS4DphC95fG
KtmhbdmCh1yQVEHCnBykD/nT6O1ldzWqfVavf7f7TKzF4IbnCE1k9f5tdwAa57sS
6cBNzb1mCyLwQe28H8J6X9d4U713WjI0HVCuZsFrN1EOZF/Mid/OP9t9BEAp8mO/
jCqGjpyqAGwSb0T0iRoWVuLQnSSE5n/FcmekrXk9mknX7LoskjOBv9FiFkQklEjx
SSZxqbalkCE0Bcv9/3rgNuiyMhyamZF3Cr1BkgsmcXRaMJhJHVJ6fioFcEmjl37c
PJo05hNO+9RtDibr3dEr/q1ZPl5NcPT2Do02SPqOsJIamGNu5Tz9oR9MguW6PKNP
AUILvlQgSeV2etobolEKxg/KB6N5r+7KVvoOjt0y50c7mS2kXsaYCJbZsj2Gr/Q+
zpMPg1JL+H66yPuVO8g7ymysWyc4xIPOdMUj0ZlpJquuWVnj4xHi2qMegZ0f2w50
fpPhGBH4WF8dn2qF/S8Jupq2mjEERPYkttYUDqUPiKH9gIZvGg73LlYiiNgHKnXY
+ENJFnbYy7Va5nIpYoNJdf76k7HaHljctMxrZC13NLdK978pfli07f/H/C6zssDj
Lf/SA5BudGBlmplm8Y15dAZxijEjdXB1Jm+F3prNhtJi7gW9UTEUdYb+fIy9sMnO
PF26Nm/gPkUQ5SiHj551XO45ET95LnMtQDDBi6aVm5aJtVrsejoHmFCTTbZcmErg
NVhtL5/cBYujk6Eng6KwvAa0wniqZTPYF0JuGNIBCuthtpWzJcemIKRVJ59C9Ogf
RklUDzeK98kgjp/6YHPnuss=
-----END ENCRYPTED PRIVATE KEY-----

Encrypt existing EC private key using the traditional format.

$ openssl pkey -aes256 -in ec_pkey_unenc_genpkey.pem -passout pass:keypass -out ec_pkey_enc_pkey.pem -traditional

$ cat ec_pkey_enc_pkey.pem 

-----BEGIN EC PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-256-CBC,3DA504A6D1C57D3EB80F4F0352F588A6

ybBslNKeNU98PGOuQaX+b+hdf0U27JqjLVDMGgHpqtr5g3huB6P0x6Guy31ranGF
Hg3V+TY7LYJqKHZ1EpfhVZk5cOKjivhDaGGI5guPS+DvkC3kmfogL/oAju5bSQTC
uVyoxGIV1lOen1SIO/g5zU5tYt4JirutrfIf4terHn8=
-----END EC PRIVATE KEY-----

pkcs8

The pkcs8 command can be used as a PKCS#8 format private key conversion utility.

Convert a private key to unencrypted PKCS#8 format.

$ openssl pkcs8 -topk8 -in ec_pkey_enc_genpkey.pem -passin pass:keypass -out ec_pkey_unenc_pkcs8.pem -nocrypt

$ cat ec_pkey_unenc_pkcs8.pem   

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg8PpYWn4Swm0wypEJ
nbdDncHddUZEzNpBaZwWxCJD6EqhRANCAAR95CxpOsM4nvXzYx4gXb8K6xZ6EUkQ
aDPo4g6jkDg0NxApMA2r7GSZiOrm/9ZwUIOA40IVeV5BUD6jsu2q+i+V
-----END PRIVATE KEY-----

Convert a private key to encrypted PKCS#8 format.

$ openssl pkcs8 -topk8 -in ec_pkey_unenc_genpkey.pem -passout pass:keypass -out ec_pkey_enc_pkcs8.pem

$ cat ec_pkey_enc_pkcs8.pem

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIHsMFcGCSqGSIb3DQEFDTBKMCkGCSqGSIb3DQEFDDAcBAgubDREXD/RogICCAAw
DAYIKoZIhvcNAgkFADAdBglghkgBZQMEASoEEDWIL671+DX/VVB1hF/2mmoEgZCA
wRUPyP8a05RTsl9BFJf9kVDY10VJGWOGhxcHlSBALqUH7OWrUCFsD+ogjHAPXcWL
P3/rSj+YeVuStMuiGJp3K/DH9V1pYHHyVFktQDvT07iNjk+DjxvwDnvCQrOLdeiN
G5rpCElTJGqVJ3GbCVqW++7MhR7ygdEnUF0vY3plcLz4usuzxKL47kglG9o/tZE=
-----END ENCRYPTED PRIVATE KEY-----