https://security.stackexchange.com/questions/59136/can-i-add-a-password-to-an-existing-private-key/59164

Of course you can add/remove a passphrase at a later time.

• add one (assuming it was an rsa key, else use dsa)

  openssl rsa -aes256 -in your.key -out your.encrypted.key
  mv your.encrypted.key your.key
  

  the -aes256 tells openssl to encrypt the key with AES256.

  As ArianFaurtosh has correctly pointed out: For the encryption algorithm you can use aes128, aes192, aes256, camellia128, camellia192, camellia256, des (which you definitely should avoid), des3 or idea

• remove it

  openssl rsa -in your.key -out your.open.key
  

  you will be asked for your passphrase one last time
  by omitting the -aes256 you tell openssl to not encrypt the output.

  mv your.open.key your.key