What? No. basemaps_mp.arena is the only .arena you can mod, as there are no others. This isn't GSC scripting anymore.
No its not gsc scripting anymore, i'm pretty sure they didn't patch any .arena mods, and also thats why you would create your own NEW one and being in a menu that is in CFG, you could then use the "exec" command so that you can "exec" a modded .arena [whole new name the one you created], and then if you don't won't a modded .arena, alls you would have to do is re- "exec" the regular mp/basemaps.arena