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Yes, in fact, it is possible to move an operating system from a physical system where it is the only OS, to a hypervisor as a guest, to another hypervisor, and even back to the original or another physical system. This functionality is not part of the basic hypervisor, but is offered through either management tools from the hypervisor vendors, or from other vendors, typically either high-availability or backup vendors.
The primary use for this is not converting existing servers to VMs -- this is usually only done once. It is more of an issue when protecting servers in a data center. For instance, a data center with 12 physical servers can be protected with an off-site secondary data center that has only one physical server running a hypervisor. In case of an emergency, the 12 physical servers can all be migrated to the single backup server, and then back once the emergency is over.