Yeah, just change the type. You can change the type of any Pokemon. You can make Swampert a Rock/Electric pokemon if you want. Poison/Steel Snorlax? Sure, go for it.
In fact, you can edit any attribute of any pokemon and make it what you want, then calculate damage on that. If you want to find out various ways to change the type of a pokemon, just read the help on Type. I wrote over 100 pages of help for the Damage Calculator, take a look at it. I'm pretty sure it tells you just about everything you need to know, and more.
FYI - Help is the big Question Mark tool at the top of the calculator. Press that...