What reason do you have to give to extend, on the website it says exceptional circumstances it can be extended?
The problem is, unless under the mileage lmits, Motability take each case on its merits.
Some things which may go for you is if the car has been adapted, or you paid a large AP which you may not be able to do again to fund a car which fits your needs very well.
However, it is really upto Motability and you need to speak with them.