Well I am winging this a bit as it’s not something that I have done, and in the case of only a few dozen entries I would probably chose to do it manually so that I could use my tags and references rather than the donor’s system. Anyway..
Import the GEDCOM to a new project.
Export this project with ‘Only people selected on Project Explorer’ (or a Focus Group if you can make a suitable one).
Merge with a DUPLICATE COPY of your live project.