Welcome to Insiders onomary. I presume you are referring to a CAT 1-5 certificate and you wish to use points or cash at a CAT 7 property? I do not believe you can do that as the certificate would only be good at a 1-5 property.
Maybe other Insiders will have more information if they've had an success in doing this.
Nope. Free night at a hotel category 5 of lower. No way to use these certificates at category 6.
Check out this list of hotels where such a certificate might be used. Re: Where can I use my category 1-5 certificate in the USA?
There are no means to upgrade a Cat 1-5 cert for any higher category property, not by offering to add points to it, nor cash, and no, Marriotts not interested in your mother either! It's an oft-asked question because whilst 2/3rds of all Marriotts properties are in Cats 1-5, they're not particularly in the kind of places you'd want to spend your leisure time, or the kind of hotels suitable for a romantic weekend break.
bejacob's thread referred to above is a particularly good place to find more salubrious Cat1-5s, but alternatively a bit of imagination can turn those certs into a good weekend away, try looking for an interesting day out in an area, then look for a Cat1-5 Marriott within an hours drive, betcha you'll find something!
That certificates a free weekend away, somewhere, so no reason to let it go to waste!