Yes, I was able to expand. I had to study the S-DD1 and how the cartridge was assembled. After that I built some routines to do the magic. Byuu helped me with some mapping and S-DD1 details and now I can do whatever I want. And it works fine in any emulator (ok, I only tested in Bsnes, Snes9x and Zsnes). And I already tested my partial translation in a real cartridge.
I'm using Byuu's debugger (Laevatein), and it's from far the best debugger for Snes. It's much, much better than Geiger (at least to me). Byuu's debugger is open source, so you can do whatever you want directly in the code, so it's the ultimate Snes debugger. You can easilly add new features and adjust the debugger to your programming style. And works on Linux and Windows.
In Star Ocean almost all graphics are compressed, and the game uses the S-DD1 to decompress. My approach will be bypass the S-DD1, storing the new graphics (probably getting it from Neviksti version) in an expanded area.
My focus right now is to finish the translation of my script and polish all issues related to the translation. After that we could try to go deep in these graphic questions, if you want and have the time. For my translation I'll fix the "yes/no" ballons and do some expansion in the battle menus, but more than that I'll need help.