If i am already checked in can I have this stay added to my rewards account?
I can't give you an official answer, but I can make a suggestion. If you have not checked out of the hotel, go to the front desk and tell the associate you forgot to include your Marriot Rewards number on your reservation and ask them to enter it for you. I assume that is the case. If, on the other hand, you have checked out, or have just joined MR, you may be out of luck. The T&Cs state the MR number must be included with the reservation. If either of the latter are the case, a call to MR customer service might help. You just might get lucky.
Definitely go to the front desk and ask them to add your rewards number. I can't imagine this being an issue.
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