Having done it to my car the other day it is possible with a good heat gun, piece of softish plastic and hammer, have your hand on the outside of the quarter panel to absorb the hammer blows like you would with a dolly. Work your way round the arch while you get someone to hold said plastic to protect paint from hammer blows. The paint just bends if you get it hot.
I used the proper tool for the job which gives a better result, I gained 15mm at certain points with mine as it pushes the whole quarter out whereas the hammer just folds the lip a little more. I had to finish the front and rear portions with a hammer due to the arc the tool follows.