which? I was able to add gamecube, naomi, and wii (all are not officially supported in Maximus Arcade)
The basics of what I did
1. I was able to get the system to run outside of Maximus (I did this before even trying to integrate the new emulator)
2. Try adding the exe and roms to another emulator you don't want to use, if it doesn't work with a another system (i.e. when i tried to use kawaks to install naomi it gave me issues, then I used visual pinball and it worked great!)
3. If this doesn't work you may have to figure out the command line, for Dolphin (the gamecube and wii emulator) I put
/e %file
in the command line, different systems may have different requirements.
4. Now you have to change the art. If you installed a new system under dosbox you have to go into the theme editor and edit the dosbox theme to match the system you installed. In theme editor... Click on an image, then go to file, import and pick an image to import. It will replace the image you clicked on. You will have to replace 3 things, the system image, the system name, and the system marquee. also with some systems you will have to change the preview image.
good luck!