There are plenty of posts regarding adding external aux valves for rear remotes. Assuming you don't want to use the expensive Kubota factory remotes, your best buy is to get a multi-spool Prince SV valve from Surplus Center and plumb it in series with the loader valve. The Prince SV will need to be configured for power beyond. You will have to fab your own mounting plate.
I have posted the instructions for plumbing this configuration in series with the loader valve at least several dozen times. Do a little searching for Rear Remotes and you will probably find the instructions.
