Not individually, but they can build the Final Order battlegroup which is a force of 10 Xystons, as can the Rebels and Underworld. This can be built on MalachorV and Rakata Prime with a Battlegroup Command and level 3 starbase.

Next update will be making many more ships cross-faction from marked planets, which will include ships like the Xyston (from Sith worlds) and Tector (from Republic worlds).

Alright, thanks!