[Dovecot] Setting a Default Domain

I'm working on migrating to Dovecot. At the moment, I have a mail path that includes the domain, so I'm using %d to get that. However, if users authenticate without specifying a domain (i.e. using "bob" instead of "bob@wiktel.com"), then %d expands to nothing. I'd like to have that expand to a default domain (wiktel.com) instead. Is that possible?

Thanks, Richard