Yes, you can add x86 or other Arm servers to be part of Mars 400 Ceph cluster. 

No matter what the new hardware is, your current Ceph cluster can add additional OSD that runs on any other hardware. The hardware can be x86 or Arm. Currently, Ceph supports these two architectures. 

No matter which server architecture, the Ceph daemons talk to each other with the same protocol.