It isn't quite not possible, at least for one refresh rate, but it is not easy.
I have had a lot of trouble getting Nvidia to actually USE the custom refresh rate with the new drivers... I found that deleting everything out of Nvidia's custom resolutions, effectively disabling that feature in the drivers, and then letting madVR do everything including adding and changing to the custom resolution worked better for me.
Giving up and using smooth motion... is not totally crazy.
