Can you convert already made buttercream to chocolate buttercream to make it easier to tint black?

I do mine... The buttercream I use is the Wilton recipe and I just had the 3/4 cup of cocoa that the recipe calls for..

http://www.wilton.com/recipe/Chocolate-Buttercream-Icing

I use SMBC, but I just add melted chocolate to make my vanilla buttercream into chocolate buttercream

When adding to already made b'cream Use a wire whip to stir in the cocoa. Use as much as you need for color and taste.

Yes...If you add Hershey's milk chocolate (melted of course) It makes it delicious!

I just add 1/4 c Ghiradelli's chocolate and mix in. Works great.