WR Long makes a kit for your tractor. I bought one from them and installed it in less than 2 hrs or so, very simple and clear instructions.
They have a true 3rd function electric/hydraulic kit, as well as a selector valve kit.
The 3rd function kit will be anywhere from $550 to $750 depending on what you want. The selector valve kit will be cheaper.
I can't answer your question about whether or why Kubota does or doesn't supply its own 3rd function kit, but here's some information on my WR Long install on my MX5100.
Mahindra 7520, Mahindra 3215HST, Case 580 extendahoe, Case 310 dozer, Parsons trencher, Cat D6,
I now sell the diverter valve that you are looking for. Complete valve kit with all the wiring and switch with generic mounting bracket. $335 + shipping. You would still need to get the hoses and whatever ends that you might need.