MoonCommander I can't say no, but at the same time, I can't say yes.
Reasons for no
The Galactic Civil War universe is quite locked in and I've already worked out all the lore and all for the Galactic Civil War. Fitting in another faction into the Galactic Civil War would mean I have to rework the Lore.
The R-UPA are a prideful faction and don't really want other people to help them take down the rebels.
Reasons for yes
This reason is a little more personal. I don't like being excluded or excluding others. Over my life, I've experienced the feelings of being excluded and don't really want to exclude people.
The R-UPA need a hand.
I have an alternative idea, however. After the period of the Galactic Civil War, the R-UPA became more open to allies after the split of the SGG into the SGG and Extrasolar.
If you became the allies of the R-UPA, I would prefer if you helped the R-UPA push the Extrasolar around instead of taking the GRF down. (Don't worry, the Extrasolar are the remains of the political side of the Rebels)