They have to have the full goalie gear on in advance and for the entire game. You'd be sacrificing a skater for that game. It's not like baseball, where some non-catcher on the roster is considered emergency catcher (generally anybody bilingual and with past catching experience).

I saw this happen in pro roller hockey (RHI) where the San Jose Rhinos' then-GM (but previously a champion goalie) had to sign himself to a 1-day contract and dress for a game.