The main requirement for live migration is CPU compatibility, i.e. the target host needs provide at least those CPU instructions, that are present on the source host(s).
If that requirement is met, and you do have some spare licenses (for at least one additional host), which can temporarily be used to add the standalone hosts to the vCenter Server environment, then the following steps may work to migrate the VM's from the standalone hosts to the existing cluster.
- create a new cluster to which will be used for the standalone hosts
- add one of the standalone hosts to the new cluster, and assign a valid license
- reconfigure networking, i.e. add vMotion networking
- migrate the VM's to the existing cluster host(s)
- remove the former standalone host from the cluster to free up the license
- proceed with the next standaloine host