Compatibility > Time Machine > How to use Lima connected HD for Time Machine
Thanks @Tim Bouhour .
But in fact there is a solution in sight. The trick is to create a sparsebundle image on the Lima Volume. Once mounted you can tell mac OS sierra (via Terminal) to use this volume for Time Machine (here is the tutorial I used: http://basilsalad.com/how-to/create-time-machine-backup-network-drive-lion/)
I tested it and it does work. Unfortunately there still is an issue: Every time you reboot your Mac the image (sparsebundle) has to be re-mounted manually. I'm trying hard to find a solution (most likely an apple script) to auto-mount the image when starting the Mac. If anyone has an idea she/he is more than welcome to share ...