wreckfest is more based on flatout 1, which was a lot more weighty than 2. it did have nitro but it worked really differently than in the sequel, just upping your acceleration rather than your speed.

from a two-minute look around the forums it looks like it's not really possible to add as a mod unless you want to fiddle with engine dynamics on the fly which seems like it could break things.

There 2 clear differences between FlatOut 1 & 2 and Wreckfest.

1. FlatOut 1 & 2 are focused on street racing and track racing. The cars are more rally and for speed.
2. Wreckfest is focused on folkrace. The cars are focused on just that.

I truly understand why Bugbear did not added nitro to Wreckfest. Sure, I miss having nitro to just slam into the AI and have a really good time (also to enjoy the damage engine), but I find it much more realistic and challenging in a good way, with having no nitro. Wreckfest have more depth that way.