Actually I need to amend that. I don't have time to look at the manual and schematics at the moment, but I think you could only access 128 patches ("voices") *INTERNALLY* and that would require firmware (OS EPROM change) and hardware (more RAM obviously).? Many of the expansions do this.
For the CARTRIDGE slot it's even worse since, as mentioned there are no more signals available across the cartridge connector so quite a few (more) hurdles would be required to overcome that.
