Unfortunately SW does not allow changing of file types (e.g. part to assy or assy to part).
Are you able to add the PEM Nuts as multi-body inserted parts?
Right now you cannot change the reference from a part to an assembly. I have had this same problem many times. I ended up just making all my parts assemblies even if there were no pems just in case I had to add some later.
Not at the present moment, but what you can do is bring the assembly
view of choice into your part drawing. You can basically have more then
one model present in the drawing.
You can use SolidWorks Explorer to change the reference model from part to assembly.
Here is YouTube video on how to do it.
I agree with Toby, you should make all parts assemblies even if there is no real assembly. Should you need to add a PEM nut later on your drawing will appear with just boxes showing where the views were. Wayne Matus post above was very interesting and educational. Thanks Wayne! I hope to remember that next time I have to deal with a modified customer part.
I regularly design sheet metal parts. I have both an assembly view(s) (usually isometric) showing where the PEM's are using balloons and the BOM but it is the part that is also in the drawing with all the required dimensions. That clears up the views to more clearly show the hole sizes without some tricky dimensioning or worse yet, over ridden ones.
I also export the part to IGES for our vendor directly from the drawing. At the end I pack and go all the required files for distribution.