How to extend /mnt/data partition?

I am trying to push some containers to a beaglebone black using:

sudo balena push local -s .

However, it is running out of space. I accessed the board with ssh and tried:

balena system prune

But still it runs out of space. So I was wondering how can I expand /mnt/data to an external SD? Is it possible to mount it into the SD card? Can I use some kind of flag to save stuff in another location?