How can I add a second partition to a Colibri T20

I’m working on a read-only root filesystem and need a second partition with read-write capabilities.
Where can I find the ubi partitioning so I can expand it? Or what would be the best way?
I would like to keep the current update procedure (, run update) expanded with the second partition.

The MTD partitioning is taken care of by U-Boot and defined in the following configuration header file.

The other part I needed was found in flash_blk.scr / flash_eth.scr