So, as I've discovered once, cars' .GEO files are 100% identical to those from the PC version, making it a potentially first NFS game for PSX to feature addon cars that aren't just texture swaps.
The .QPS files' mystery has also been solved - a slightly edited FSHtool, which I named "PSHtool", flawlessly converts them to .PSH files, allowing their modification.
But here comes the braking point: the only thing that keeps me from throwing in a first new car (Ford Mustang Mach III, to be specific) I've even prepared textures for is a lack of program that could build a .PSH file from ground up: Leo's PSH editor only allows replacing TIMs with the exactly same size, and PSicture is just a texture viewer with no import function. Other .TIM editors I know don't support PSH files. The only solution I see - replacing .BMP support of FSHtool with .TIM support - requires knowledge of programming languages, which I lack.
Other files identical to their PC counterparts:
(Z)TR0*.QPS (not PSX texture files, these are ZTR00*.QPS)
Some files lack any counterparts (like Z%CAR%.QQS), probably thanks to different platform architecture; some are only slightly different (like, .MAP files only have different ends).