If I understand correct you want several locations for your ppu files (kind of realtive to the pas file)
The only way I know to do this, is by puting all the units from other places into packages (one package per place).
Compile the packages, use the packages in your project.
Compilation will be faster, as the files in other places are compiled, and don't need recompiling all the time.
But on the other hand, if you edit one of the file in the other places you have to recompile the package, since it doesn't get re-compiled with your program.