Can I add peanut butter to cook and serve pudding to make peanut butter pie? If so, do I use less liquid?
Welcome! Per allrecipes , yes you can, and no change liquid is needed. The recipe in the link calls for a pre-made graham cracker crust and uses 1 cup of peanut butter.

The instructions read as:


  Prepare cook and serve pudding as directed on package. Stir in peanut butter. Bring mixture to a boil and pour into graham cracker crust. Allow to cool.


If you do a Google search, there are also many recipes available that use instant pudding.