yes you can, but since you already have existing running vm setup already, when you adding the 2nd, 3rd nodes to the cluster, you must first make sure /etc/pve/qemu-server/ and /etc/pve/lxc/ has no config files (move all *.conf to /root/qemu-server and /etc/lxc/), then also make sure all your VM ID does not duplicated. for example, you can't have VM instance on NODE A with ID 101 running and also another VM instance ID 101 running on NODE B