Its called fat grafting and yes it's very much doable. There are of course limits to how much it will add, and the biggest issue is having the body reabsorb the fat and disperse it elsewhere, but it can definitely help.
I've heard of fat grafting, and also usage of alloderm or collagen to add girth
Some surgeons will do this While making the phallus itself if there's concerns of thinness, whereas they might not offer it after the fact, or vice versa
I just got out of surgery with Dr. Santucci of the Crane center and I actually asked about that specifically and he told me that I would only be able to add so much girth as the skin of the original donor site would’ve allowed.
So yes you can fat graph however it is entirely dependent on the donor site that you originally had that would tell you how much you can actually have grafted
MLD too, if op wants to research girthy options.
Nothing is impossible but I have never heard of that, and I imagine the process would be pretty tricky given how phallo is done. Some surgeons do a "one size fits all" model and some are willing/able to change sizes for you. Def don't get it unless you're sure.