I'm sure you could get a set of 'snap-on' or 'clamp-on duals for it as they are pretty universal. However, in every Kubota manual I have read (from
B7610 up to M9540) they specifically recommend against dual wheels, my guess is it is the same for the
M135x. Generally, tractors that are designed to run duals will have rack-and-pinion rear axles where the axle itself sticks out to mount the duals on, mostly row-crop tractors 100+hp. (the Kubota despite it's size is still a utility class tractor in many regards.)
I'd attribute the stern warnings in the manual to the fact that they didn't have duals in mind when designing the tractor. As such they don't want the liability that would go along with it if there was ever trouble/failure. What are you using the tractor for that you need duals?