Can I add meringue powder to already made icing? I have a 16 oz can of ready made icing that I would like to use as royal icing. How much meringue powder would I add to 16 oz of regular icing?

I'm not sure that you can use ready made icing as royal icing. Any grease causes royal icing to break down.
Royal icing is made of meringue powder, powdered sugar and water.

You cannot turn a can of ready-made icing into royal icng. They are two completely different types of icing.

make your r/i with the meringue powder...water.. powder sugar.. do not add it to all ready made icing.. even if you made the icing yourself.. please!!!

Ok...thank you everyone....I just made a homemade batch of royal icing!! I just needed to clear up the thought of taking the short cut.

Well, using meringue powder is a short cut.
(No cracking eggs or separating egg whites from yolks, and hoping you don't break a yolk.)

WAIT! RI needs a touch of acid, like a few drops of lemon juice or aetic acid.

If she used Wilton meringue powder, and the Wilton R/I recipe she should be fine.

http://www.wilton.com/recipe/Royal-Icing

The Wilton egg white R/I recipe does require cream of tartar:

http://www.wilton.com/recipe/Egg-White-Royal-Icing-1