You can get nightmare modules for a variety of games including Shadow Dragon here:
As for trying to edit the save rather than the ROM, no one has ever even managed to edit GBA save data, let alone a less understood game like FE11. If you want Samson AND Arran in a post chapter 16 save, might as well just cheat by editing the ROM to have him join you in the disposition editors and then once he’s in your party he should be stored in the save so you could use that save with another ROM.