Thanks for the tip, it seems to work (its copying now)
This is how to do it:
- wget https://fp2.retsifp.de/signed-recovery/twrp-3.2.1-0-FP2-signed.img
- fastboot boot twrp-3.2.1-0-FP2-signed.img
- adb shell dd if=/dev/block/mmcblk0p20 bs=19999 | dd bs=19999 of=p20.img