As there was no option visible in the GUI, I looked into using adb. The following seems to have to have done the trick:
$ adb shell
FP3:/ $ sm list-disks
disk:179,64
FP3:/ $ sm partition disk:179,64 private
FP3:/ $ sm list-disks
disk:179,64
FP3:/ $
Then the SD-card becomes available as “internal storage” which should be encrypted (going to test this ASAP)
Hope this helps others as well.
Best regards,
Felix