Raider is a tool to automate linux software raid conversion

raider was working on proxmox 3.x witout problem i have not tested it on 4.x but i suspect it is working
 
Besides, Proxmox VE does not support software raid (as of mdadm) at all. So you're on your own if problems arise. If you want to use software raid, the only working (and supported) solution is using ZFS (or Ceph/DRBD for networked storage).