You might be able to roll it out, which basically consists of getting a two blocks of wood that have a hole in them the exact shape of the tube and sliding them up and down the tube. The idea is that the dent deformed the tube outwards elsewhere, and by rolling the wood over them it will force the tube back into the original shape.
I have no experience with this at all however, but see if you can find a shop or something that does.
You need to get the area clean and roughed up before you apply this.
You mix a hardener in with the putty, then you have a pretty limited amount of time to get it in place before it sets up.
You need to sand it down flush with the frame once it sets, so don't apply huge gobs of it or you'll be doing a lot of sanding.
Have fun. Bondo has a great smell.
Edit: Others probably know more about this than I do, but I would imagine that filling the dent with Bondo won't replace the strength you may have lost due to the dent. It's a lot of sanding/painting/sanding to get it to look really nice. Perhaps just leave the dent alone?