You can do something like this:

1) make "sdl" directory in your units dir
2) copy all the pas files you need (sdl.pas, sdl_mixer.pas etc.) to this dir

Run freepascal and add:

c:\my_projects_dir\fpc\units_dir\sdl
c:\my_projects_dir\fpc\units_dir\sdl/*

Remeber to keep dlls in your program dirs