It's possible, though I suppose that's debatable. But most of the surgeons doing metatoidoplasty do recommend pumping. It's not the gains you actually see from pumping that you would get. It's more that you increase blood flow at least theoretically allowing growth. You have to do it fairly frequently (at least daily, perhaps more than once a day and at least 5-15 minutes per day).
Always be careful. Stop if there is pain, go very slowly (and yeah I agree use lube).
Did I have permanent gain, not sure.

--Jay