Technically no. The reservation is suppose to be in the name of the MR member and paid by the MR member.
Now, for reality, you can make the reservation using your MR # and add your fiancee as a guest on the reservation and she should be allowed to check in telling the front desk you are arriving later. Some places will be sticklers and request you stop by the front desk upon arrival.
I'd suggest getting her signed up. This stay alone will get her Silver status and some minor perks. Swapping some nights when you travel together can build her status (at the cost of yours) and you can usually get the Plat perks if you show your card at check-in.