I talked to an important italian medician who make foot filler. I came to her office and she said me about the possibility to increas 1 cm, with hyaluronic acid.
Just 1cm but it's possible to make others operations to increase another one.
No more, although at phone or on her website, she told about 6cm.
6 CM in your feet would be a terrible idea. It's possible to do a few CM into your feet, but I think it would be awful. I would rather go through LL to get a permanent 5 or 6 CM than do something temporary like that.