Tools and documentation (updated 18th Aug 2019) https://www.dropbox.com/sh/392xi56ys1uq ... ?dl=0&lst=
You’ll also need Nightmare https://www.romhacking.net/utilities/1307/
I’ve wanted to make an S5 editor for a good while now, and thanks to Atonias’ post a while back was able to figure out a great deal of how the game tends to store its data. But eventually I opted for the path of least resistance—namely instead of reinventing the wheel, just making the game functional with an utility that’s both extremely convenient for fixed-length tables and one I already knew how to work with.
Modules included as of now will allow you to edit…
Enemies
Character base stats & stat growths
Character elemental affinities
All categories of equipment & accessories
Spells
Skill effects
Equipable skills
Weapon growths
MP growth requirements
Rune & healing item prices
Hopefully there’ll be more modules down the line as I hunt the addresses down (rune slots and formations most pressingly), but these should be enough to warrant a preliminary release. Obviously these will never be capable of very sophisticated modifications; both the scope of Nightmare itself and my level of knowledge are limiting factors there.
Instructions
Open S5tool.exe. After you've recovered from the awe of the expansive GUI, choose 'Extract' and navigate to your ISO (the program is now set to work with both NTSC-U and PAL, but I can't speak for the Japanese version). The program will let you know once it's done, and will have spat out an 's5_extract' with no file extension. Using Nightmare and the modules provided (shouldn’t be too difficult even if you’re unfamiliar with the program, but I’ve provided a short tutorial in the readme) make all the edits you wish to the extracted file. Once you’re satisfied, open the tool again and pick ‘Insert’. You’ll be prompted to first choose your ISO, then the modified file. If you see ‘Inserted!’ pop up, all went accordingly.
Huge thanks to Atonias for finding most of the addresses. If anyone has any questions or finds any mistakes I may have made (and I most probably have), do throw them my way!