Per the above post. This is how I'm planning to do my Pumpkin Porter.
If you do a partial mash in the first step you put the baked pumpkin along with some 6 row malt you will be able to convert some of the sugars in the pumpkin along with the color, aroma & taste.
Also note as in cooking I prefer to use fresh spices whenever available, but in this case you will want to be careful not to overwhelm your beer (especially with fresh nutmeg).
A great resource for doing your first partial mash is John Palmer's Howtobrew.com or there is a
great sticky thread in the beginner brewing section called Easy Partial Mash Brewing
My recipe (adapted from Palmer's Porter recipe)
6 lbs. of Pale Malt Extract (syrup)
1/2 lb. of Chocolate Malt
1/2 lb. of Crystal 60L Malt
1/4 lb. of Black Patent Malt
30 oz of Baked Canned Pumpkin
1/2 lb of 6 Row Malt
1 oz of Nugget (10%) at 60 minutes
1 oz of Willamette (5%) at 20 minutes
Pumpkin Spices (1/4 tsp of nutmeg, cinnamon & all-spice) at 5 minutes
Nottingham English Ale yeast (washed from a batch of English Bitter)