Welcome to FlyerTalk, Emskevin.

Not at check-in. The hotel can't do it. Only WyndhamRewards central can make points reservations, not individual hotels at the check-in desk.

And you can't convert an existing reservation from cash to points AFAIK. You have to make a new reservation with points, and then cancel the cash reservation. And to avoid cancellation penalties, you have to do all this before the cancellation deadline. At most hotels for most of the year, the cancellation deadline tends to be about 1 day before arrival, but check the specifics, because that cancellation deadline can be very different for some hotels and/or during some events. (For example, whatever city the Super Bowl happens to be in, expect much longer cancellation deadlines during Super Bowl weekend!)