This may be a silly question, can I add peanut butter from the jar into a butter cake batter and cook it? Will that work as a peanut butter cake? I am not sure if doing that is going to change how the cake cooks or rises etc? I just have to make many of them and i have many packets of butter cake at home which need to be used!

It changes the fat content in the cake. Since you are increasing the fat it would make it more tender and could affect the bake time. This thread http://cakecentral.com/a/peanut-butter-cake-2 she gives a recipe on peanut butter cake with a boxed cake mix. She used french vanilla but I am sure it would work most any vanilla or yellow cake mix.