Use of Bonus Cards in Play

I have several lovely Munchkin cards that I got with purchases from Warehouse 23, and I'd like to know if there's anything other than barefaced chutzpah that allows me to introduce them into a game. If when it's my turn to draw from the deck I instead pull one of these beauties out of my sleeve with an evil laugh, is there any rule or ruling that allows me to do so or do I have to brazen it out myself? Not that I have a problem with that, but if I'm bluffing I need to know I'm bluffing.

Use of Bonus Cards in Play