You can't easily add bezels. The only way is to replace the ones on the device but that requires unpacking the firmware, replacing files, re-packing and flashing back to the unit (not the easiest process tbh).
You could make a script to dump the emulator folder to the SD card and read it from there so that it will read your own music and bezels from SD. It was done for the Atari Flashback 9 here. I'd think you'd just have to edit this script to point to the correct folder for the Genesis Flashback.