Technically, it's possible, but I would refrain from doing so. Planet Coaster is requiring a very specific setup for it's custom model files, which it's easy to say, no free models you find anywhere on the internet will have it setup to work with Planet Coaster's model verifier. You're better off learning a 3D program and then giving it a try for yourself. Also you could always ask someone to model for you if they're taking requests.