The reason it's currently not possible to use FSR 2 and 3 is because VaM has incomplete motion vectors, if I ever find a way to "repair" the motion vectors that get generated, then I plan to add FSR 2 to
https://hub.virtamate.com/resources/post-processing.37521/
FSR 1 would definitely be possible as is, but personally I don't think it looks very good, it has a lot of image instability because it doesn't make use of temporal data, it's purely a spatial upscaler, so although I'm sort of curious to add it, I'm much more interested in fixing the motion vectors because it will unlock a lot more possibilities.
If I ever get 2 working, 3 should be easy also because they use the same data.