I don't recommend substituting cooked fresh squash or homemade pumpkin puree. Commercially canned pumpkin puree gives consistent results. Fresh squash varies in moisture and starch content, depending on the breed of squash, the age, its growing environment, and other factors. Your pie filling may turn out too dry and dense, or on the other end you could end up with a runny pie. If you would like to attempt this substitution, please proceed at your own risk.