Yes, you can add pudding to any mix that already has pudding in it.
Before CC, I baked a cake for my family (my husband hates cake, because it is always dry no matter where he gets it from) That being said, I baked this cake, Chocolate cake... (already had pudding in the mix)
I added 2 boxes of butterscotch.... and the cake was very yummy... The cake was sort of Jiggly because it had so much pudding in it, it was extremely moist, and my family loved it. However --
Now that I know a little more, I could have just added the 1 box of butterscotch for flavor and texture, butter, and milk and I would have gotten a much better cake, that I could ice, and decorate much easier.
As for how much butter- It is melted - and it is normally half a stick to a stick of butter ... I sort of feel and taste my way through it. ... When I taste a batter and it is really good, I know it will be a good cake.
I used the Wasc recipe as well and added pudding - My cake was more like pound cake, but there was not a dry mouth in site...
Hope this helps!