is it possible to add a custom mission on ps2 disc? like in instant action? if not can i edit a default instant action mission for the ps2?
You can only edit PS2 files once they are taken off the disc, then you must recompile it back into an iso file.
If you put it on a disc, it will not work unless the PS2 is softmodded (plus if you use the disc method you're going to waste a lot of discs). Unless you have done this, it will only work on an emulator.
so basically im using a ps2 disc on the ps3. does this change anything?
Nope. Still needs a modded console and the rest still applies.