No. A free night award is listed as a 25,000 point hotel or less. I was aloo told that all cat 5 hotels are now listed as cat.6 and you will need 35,000 point. Those that did the Marriott upgrade of your Marriott credit card will have their free night upgraded around Dec, for 35,000 points. I just called 1/22//19 trying the same results.

Redeem Cat 6 room with 35k FNA certificate + 15k points
 

Based on anecdotal evidence on the interwebs I would say no.

I have been told that it is technically impossible for a Marriott agent, including an Ambassador, to execute because of the way the Free night certificates are applied. "Some" hotels may have a workaround if you deal directly with them. YMMV.

Too bad MR does not follow Best Western's lead. Back in the 00's when BW still used paper certificates, they offered Level Adjustment Vouchers to add to existing cert.s to make up the difference between a lower-level award & a higher-level hotel. Perhaps Bonvoy should add this feature; it would be a true, non-sarcastic Enhancement!