Combine 2 drawings into one

I designed a case for a keypad, and a cover using the shape from the case bottom. I forgot the holes in the case bottom, and want to put the two parts into a single drawing to project the holes to match on the bottom part. When I “import” the top into the 1st drawing, the original part is replaced. How do I get both models in one file? I know I did it before, but can’t seem to get it to work this time.

Not sure what is going on with the losing part of your drawing on import. But a simple solution could be to create a sketch and export that as a DXF that locates the holes and import that. Or export as STEP and import that into the drawing. Maybe that will get you what you need.