If you are talking about automatically setting up a user's identity the first time they login to Roundcube then have a look at the new_user_identity plugin which is shipped with Roundcube. It pulls information from LDAP. If you don't store your user information in LDAP then you could make your own plugin based on this example which pulls the info from where ever you do store it.
Another option is the new_user_dialog plugin also shipped with Roundcube. This prompts the user to setup their identity on first login.