hi,
this thread might be of help:
Using qemu-img you can convert the images to ceph-rbd format, see:
http://docs.ceph.com/docs/hammer/rbd/qemu-rbd/#running-qemu-with-rbd
If you have just a few images, convert them manually and update your templates with the new ceph-images, then re-instantiate your VM’s.
hope this helps!