I bought a bucket of Sam's club buttercream. Can I add more powdered sugar or something else to make this crust over when icing a cake?

The but-r-creme (in the round bucket) will crust when icing a cake without anything being added.

This bucket doesn't. I even poped the cake in the fridge for about 30mins and I still wasn't able to smooth it like I do with my homemade buttercream.

I don't know, then. Did you have the round bucket or the square one? I don't think the square one is supposed to crusted, it is a whipped buttercream. The round certainly should.