Yep, it's possible. Teancum here from Gametoast -- I've been working on it over the last week and have the following working:
1-New modes on existing maps (Hero CTF)
2-Custom heroes (currently Kyle Katarn is the hero on GCW Coruscant Conquest)
Now before anyone asks I can't write up a simple tutorial. I worked with the mod tools since they first came out for PC for SWBF1, so I know the ins and outs of them. BUT, I might just release my custom content builder (it's just a custom "map" for the PC version modified to build Xbox content) so you can use it with the PC mod tools.
Tonight I'm experimenting with new maps - I'm rebuilding Bespin Platforms from our PC source file conversion to see if custom maps will work on the Xbox. It'll probably take a week or two to figure out whether I can get it to work, so no ETA there.
Another thing I should point out is that every bit of this mod comes from original source files. What I mean by that is this: There are a few custom Jedi Outcast converted models, but these were converted from the original, publicly released source models. Same deal with any SWBF1 content. I won't convert anything that isn't already "open source". So please don't ask for something like Master Chief as a hero - that's not happening.
That being said there's no ETA on the mod. But if you want to keep up-to-date check out the link posted above.