Yes, you can fill instead of soak off every time. I do the color over the white (usually bubble bath, hawaiian orchid or passion), so when the client returns,I filing the top coat, most of the color and the white off, leaving mostly just the clear base. The trick is to really make sure there will be no fill lines with french, which can be challenging with a soak off gel, considering the rubbery nature. Don't use anything lower than a 180, otherwise you will just loosen the whole thing and it will peel right off.
It took me awhile to get the fills to look as good as the fresh application, but as long as you file zone 3 flush to the nail and concentrate your fill product there and just lightly feather over the reast of the nail, it'll look as good as new!