I've thought of doing this before, but decided against it. You would need to make sure the powders are very uniformly mixed in order to get consistent dosing. This can be harder than it would seem with powders, especially if one powder tends to stick to the other.
But if you really want to try, don't just dump the dbol in and mix. Add a small amount of the dbol (maybe 250 to 500mg) to the protein, mix very very well, then add a bit more, and again mix. Repeat until finished.