you can include a header file from the object (relative to objects location), and then you can do includes in that relative to that header files (as with normal C)
what you can't do add paths to the include path.
obviously, this extends beyond include paths, since to have separate C files, you would also need to be able to compile these or add libraries to the link path... which is what the new modules stuff is all about.
I refer you to my push object where you can see what I did to workaround this.
basically my objective was to structure the code, such that when we developed 'compile units' I would be able to transition the code simply.