I used to do this in Inventor all the time, creating a "weldment assy" of several parts, then importing (or inserting) that assy to make a new finished machine part...

I see in SW how to "Insert Part" in order to derive one from another, but as the weldment is an assy, it doesn't see the file. Also, I see that I can save the assy as a part file, but then making changes to it later isn't allowed (or at least not that I've seen)...

As always, your assistance is appreciated!

(Ref below)

Weldment assy (4 parts):

Want to make a single part (not assy, as the weldment is now a new single part) from the above that would be the finish machined with a new part number