I have so much left over buttercream (butter and shortening) - instead of having to make a batch of peanut butter buttercream (which I've used a recipe off here) Is it okay to just add some peanut butter into my already made batch of regular buttercream? I'm googling now, and not finding much an answer, so I figured i'd ask on here also...

Thanks!

I do it all the time! sometimes I also add cream cheese along with it to cut down on the sweetness.

I have done it twice with nutella.  Turned out great.  I think I added in a little more liquid with the nutella.  I just made some peanut butter icing for the first time and mixed the PB in with the fats first then completed it.  But if I had already had some plain icing on hand, I would have given it a try to mix it in.

Yup- do it all the time!  I just add peanut butter to taste and some extra liquid (I use milk or cream).