You do this in an assembly.
This morning, for instance, I needed to add a small cast-iron access door into something else. The way that I did this was to create an assembly, and position the small access door (the actual door, I mean) where I wanted it to go on the larger object. Then, editing the larger part while still in the assembly (that part's important), I projected the outline of the access door onto the larger part, then offset it for clearance, and extruded a door-shaped hole into the larger panel. If I make changes to the shape of the access door next week, the hole cut in the panel will automatically update to match the new shape of the door. (Well, usually it will - the connections can be quite fragile, and you don't want to screw them up.)