While this would be one of the single most coolest things in the game but I don't think it would be possible. Since you have liberated Borg thats understandable because the Borg was once human and probably doesn't want to be a slave but you cant exactly turn an Undine like that and you would sort of have to capture one first since they only serve them selfs.
There must be some dissident Undine, after all, they're so stubborn in whatever their point of view is. It could be an older Undine who remembers Janeway's Treaty. Also, instead of holding a weapon, giving the undine a weapon would just color its normal arm blast with whatever type the weapon you gave it.