unions for PEX to coper
The big home stores (Lowe's/Home Depot) have plastic compression unions for what you want to do. If you dislike the plastic they have brass compression fittings for the copper, which you can join to a PEX fitting, either with a close nipple or use male threads on one fitting screwed into female threads on the other fitting. Other hardware and plumbing supply houses will have them as well.
Hope this helps, John