Do you already have a ceph cluster installed and operational? And a ceph pool for the instances? If so, with the pool accessible through cinder-volume, you can just designate where the volume is stored explicitly.

Here's some documentation with commands to run assuming you are using Rocky: https://docs.openstack.org/cinder/latest/admin/blockstorage-volume-migration.html (https://docs.openstack.org/cinder/lat...)