How do i stiffen store-bought frosting?

I am making a scrabble cake for church on Sunday. I already have a lot of store bought frosting that i want to use up, but i want to know if it can be successfully stiffened so it will not run or get soft. I am thinking the answer is powdered sugar, but i don't know what ratio. The stiffened frosting will be used for the cake and then i will dye it and use different colors on top of chocolate covered graham crackers. After that i want to write some stuff on the frosted letters.

-0kpokopo <----- (that is my 2 yr. old's input!)

I am making a scrabble cake for church on Sunday. I already have a lot of store bought frosting that i want to use up, but i want to know if it can be successfully stiffened so it will not run or get soft. I am thinking the answer is powdered sugar, but i don't know what ratio. The stiffened frosting will be used for the cake and then i will dye it and use different colors on top of chocolate covered graham crackers. After that i want to write some stuff on the frosted letters.

-0kpokopo <----- (that is my 2 yr. old's input!)

Becky, don't worry about a ratio - just put the powdered sugar in a half-cup at a time or so, mixing in between additions, until you have the consistency you want. But keep tasting! Don't make it too overly sweet.