Can Master Encryption Keys be entered by Key Officers who did not configure them originally?

Yes - as long as the passphrases are identical (they are case sensitive) to the original system and the Key Officer has the proper authority. 

The following users can load the Master Encryption Key

• QSECOFR user profile (unless excluded in the Key Officer settings)
• A user profile with *SECADM authority (unless excluded in the Key Officer settings)
• A Key Officer that has a *YES specified for the “Load MEK passphrase parts” authority setting

The following users can set the Master Encryption Key

• QSECOFR user profile (unless excluded in the Key Officer settings)
• A user profile with *SECADM authority (unless excluded in the Key Officer settings)
• A Key Officer that has a *YES specified for the “Set and clear MEKs” authority setting