How can i add extra units like ARC_Commander in his Battlefront Xtreme i mean in the map's that already are in the game like Felucia, Coruscant and all the others

Pretty sure he would have had to edit the LUA's then munge the shipped maps again, to add extra units.
Hex-editing would only change the existing units.

You want to munge your own mission.lvl to replace the game's mission.lvl. All the map scripts are in that file. Edit the retail map scripts, create a new mission.lvl, and replace the game's (after backing it up).