I couldn't find a recipe for Duncan Hines but I did find one for Pillsbury Frosting. I haven't tried it.

Pillsbury frosting clone

2 lbs. Powdered sugar(confectioner's icing sugar)
1/4 cup Coffee Mate (powdered nondairy coffee creamer)
Dash of salt
1 1/3 cup Crisco shortening (solid vegetable shortening)
2/3 cup water
1 Tbsp.vanilla extract
1/4 tsp.butter flavoring

Mix by hand until fine, then mix in mixer on low. Add water, vanilla, butter flavor and mix well. Then mix on medium until creamy.

You can substitue 1/2 of the crisco with butter but you won't get a white icing, you'll get an off-white or ivory color. Also using the crisco, the frosting will be firmer and won't welt or weep like it could with using butter.

For Chocolate add a bit of cocoa powder.