If its on ur property then you can. If there is grass or weeds under the pylon then the land is good for cultivation. Just dont let runner beans go up it and dont harvest during lightening!
Just make sure that you don't plant over anywhere where access may be required to the pylon eg people going up for maintenance or moving, testing etc... so they don't get trampled upon.
Pylons are zinc coated I think and this can affect the soil round the pylon so you might want to check what you can and can't plant there for it to survive.