Is there a way to play Munchkin Quest with 5 people without the expansion?
There is a rulesheet for expansion here.
I took a glance on it, there are some special expansion-specific things, but I found nothing concerning gameplay changes, so I suppose you can buy (or make your own) munchkins and stands for the monsters of the appropriate colours and enjoy the game :-)
I can't recall any rule prohibiting it, and, as rules usually say, "Like we could stop you" ;-)