Welcome to the forum. You can't create a mirror of the part such that it would create solid features in the assembly...unless you only have cuts. If you don't want to create another part, then you can perform a mirror within the part itself. Either activate the part or open the part and then go to:
Operations (model tab pulldown) -->Feature Operations-->copy-->Mirror,All Feat, Dependent, Done --> pick your datum for the mirror.
You can also mirror individual features using the mirror button but it may have unexpected results on some kinds of features. Does that get you the results you want?