Create a new account for the special callsign, that is the only way.
If you do it via the My Accounts screen, the credits from both accounts are combined for the awards program:
http://www.eqsl.cc/qslcard/MyAccounts.cfmBottom of that page "Register a New Attached Account", change the callsign to be the contest callsign and press create new.
73, Dave - VA3KAB