Yes, it's possible to that but it's not so easy as this device is running Linux (no ADB support) and the partitions as read-only:
1) You will need to create your own rom partition and update file, or flash it using AndroidTool
2) The partition is SquashFS, this is a compressed read-only format for Linux. You will need to have a Linux machine or install the Linux subsystem for Windows 10 to create a new partition using mksquashfs
3) The new partition should not exceed 60MB. Since it's a compressed format, it may be larger, but the .img file to create the upload for flash should not exceed 60MB (61,440KB). If the partition is larger, it will break the 'next' partition, which is the /emulator, soft-bricking the device. You can fix it flashing back the original rom and emulator partitions using the right parameters
4) Add or remove games is quite simple. There is a all-games.ini file, you only need to edit it
This is only an overview of the process but it's not that difficult to do if you have the tools and some patience.
Replying
@D-an-W and
@kublai:
This device has several partitions, being the following the most significant:
/rootfs -> Base OS files (14MB). It's the same for all consoles (Genesis FB, MD FB, Legends FB)
/rom -> Where the ROMs, box arts and gamelist are stored (60MB)
/emulator -> Menu (retromenu), Emulator (retroplayer) and cores are stored in this partition (8MB in Genesis FB/MD FB, 10MB in Legends FB).
My custom firmware replaces this partition. It has a change in retromenu to read all extensions, and/or replace Genesis Plus GX core with Picodrive from ModMyClassic
/data -> Has control files (retroplayer.ini, bgm_config.ini, play-data-recent.ini and sdcard_config.ini) and save states/sram files.
Atgames update replaces this partition, writing a new sdcard_config.ini to enable SD Card. This partition is the same for Genesis /MD FB. The size for this partition should be around 160MB but it looks like Atgames made it smaller inside the OS; The explanation is that the OS creates a /tempfs partition for temporary cache. I didn't see the actual size in Genesis / MD FB but in Legends FB this partition has only 4MB and the rest of the NAND space is available as 'tempfs' to decompress the ROMs.
This thread have the links for the EU/UK firmware. I edited the other thread with all links but I will do the same with the post in this thread as well.
Sorry for the lengthy post.