I was thinking this same thing earlier today. I want to do my regular Hefe recipe and just add pumpkins in the mash for an original take on a pumpkin beer. Seems like a good idea but I've never heard of anyone doing it.
I don't know that it would still technically be a hefeweizen if it had pumpkin in it, but it seems that there are a few recipes for pumpkin wheat beers that are quite successful. You probably wouldn't need as much pumpkin to get a prominent flavor, as you would with something like a brown ale.