Can stainless steel be welded to fenders and quarter panels?

You can weld it with GMAW (mig). I did a lot of this with 316L wire and trimix gas (co2, argon, helium) at the shop I used to work at. I'd imagine it can be done with TIG as well, but I'm not very experienced in that area.

Stainless warps a bit differently than mild though, so you really need to watch your heat.

Honestly, I'd just grab a piece of mild steel and use that rather than setup a welder for stainless just to do this one panel. That is, if you're dead-set on welding it.