rockstor works just fine on a USB3 stick. In fact if it detects that it’s installed on a USB, it will limits the write to the USB.
The next part of your question is not very clear to me. If you have one PCIe 1x slot left then you can definitely find a PCIe 1x to M2 adapter for your card (SATA interface M2).
If you want to make an image of your m2 card then use dd
Make an image of a m2 drive in /dev/hda:
dd if=/dev/hda of=~/m2disk.img
Restore the image to /dev/hdb:
dd if=m2disk.img of=/dev/hdb