Not sure what you're asking here. Do you mean use points for a reservation, turn an existing paid reservation into a points stay, add to a paid reservation with points, or something else altogether?
I don't believe that you can "convert" an existing paid reservation into a points stay either in advance or during the stay itself, other than cancelling the original prior to checking in and rebooking as points. A paid stay can be done as a combo paid & points, but I think you have to book it as such in advance. A reservation can be made with points for anyone other than the member, but only over the phone. I've done that for friends of mine a few times. Very easy, but the online booking engine doesn't allow for names other than the member, hence the phone reservation requirement.