UNIX users who do not have valid user accounts in the default Kerberos realm can be automatically migrated using the PAM framework. Specifically, the pam_krb5_migrate module would be used in the authentication stack of the PAM service. Services would be setup up so that whenever a user, who does not have a Kerberos principal, performs a successful log in to a system using their password, a Kerberos principal would be automatically created for that user. The new principal password would be the same as the UNIX password. See How to Configure Automatic Migration of Users in a Kerberos Realm for instructions on how to use the pam_krb5_migrate module.