I use box mixes for the wedding cakes I do, I am not a professional, however. I add any extra flavorings to the water, to keep the proportion of liquid correct.

My recipe for frosting is:
1 lb. confectioner sugar
1 cup crisco
Butter flavoring
Vanilla or any other type flavor I want.
I also thin as needed with milk.
I do not have an amount for flavoring, I add by taste.
I hope this helps.