So I noticed STR Mods is already at it, making mods and such. I decided to do some simple testing myself and found out a few things:
1) I noticed STR Mods has a working Santa and lawnmower, so I decided to simply drop the American_01 in the vehicles folder and saw that it was added as another character in the character select.
2) Stuntfest has the gamemodes from Wreckfest in its files, which are most likely just leftovers of the copy from Wreckfest.
3) Modding in new cars is as easy as Opening "Career/default" in BagEdit (thanks Bugbear!) Problem is that the sounds for the American 2 didn't transfer, so I'll look into that.
4) Stuntfest already has mod support (which is rather obvious since it's using the same engine as Wreckfest, still a cool find though) simply add a mods folder to your main directory, like in Wreckfest.
5) Resources from Wreckfest can be used in Stuntfest, although currently has some compatibility issues (ground textures, engine sounds).
6) Even the Throw-A-Santa is compatible, however took a little more effort to place the art files in the right directory.
7) Stuntfest currently has a max allowance of 8 vehicles.
9) Despite being the most closely related vehicle to Stuntfest, the American 1 crashes on selection even when using the same settings as other modded cars. Reason unknown.
More info to come as we find it.
EDIT: Removed some pics, rewrote some facts.