We sell many brewhouse kits at our Brew on Premise and have some customers that have played around a bit with fruit and other additions. As for the pumpkin beer...use a red ale or cream ale kit. Roast your pumpkin at 400-425 with some brown sugar, nutmeg, cinnamon, and cloves until nicely caramelized. Add straight to your fermentor on day one, probably in a mesh bag for easy removal. Make sure it is freshly roasted and had just enough time to cool before adding wort and yeast. Remove when racking into secondary. That is the method that customers have had success with. If you have access to Ringwood Ale Yeast #1187 or one of the American Ale strains #1056 or #1272 from Wyeast they will provide you with the best flavour possible. Hope that helps.
Edit: because you aren't boiling the wort. Smaller chunks is better as to maximize surface area and flavour intensification.