That's kind of right, but you'd need to modify files to make the content appear in game. Its kind of a difficult question to answer.
If you're trying to add the vehicles to one mod, you'd copy the files from the first mod and place them in the same folders as the other. Then you'd have to update rewards.mfk and level.mfk and leveli.mfk to load them when the mod starts.
If mods do not specifically support each other and use the same files or file redirections, they are not supported. You can modify and combine mods by decompiling them and fusing them together. However the process could be lengthy and varies depending on what you're combining.
If by concept art you mean modeling, then I could let you work on my mod.
A lot of people who want to work with others generally want to have a second person to lean on. So finding your niche in modding is super important in my opinion. Lean on each other and create awesome things.