Here is another way to do this and it's in one file.
1. create the part (say a casting) to the point where it's complete, then extract a body at that point with time stamp enabled and name it say "casting"
2. add another feature, say a milled surface, and extract a body at that point also with time stamp enabled, and cal it "milled surface"
3. add drilled holes for the finished part
4. create a reference set for each of these three solids and add only that solid to it
Then when you add this part to an assembly simply display the proper reference set. There are multiple solids but everything is in one file and all parameters are intact.
NX10.0.0.24 MP1/Windows 7 Service Pack 1