It's been a while since I used Inv5, but I think this will work.
Activate the part into which you want to place holes. Then using 'project geometry' button, pick the round profiles of the holes you need from the greyed out mating part(s). When you return from sketch mode, you can use the cut command to make the holes (will only be rounds). Or, place hole center at center of each profile and use hole command to create.
Hope this works for you.