In some previous versions it was possible to add hosts with a free Hypervisor license to a vCenter Server in evaluation mode. However, IMO this was probably more a bug than a feature. If your two hosts are still in the evaluation period (60 days) you can reset them to evaluation (i.e. unassigning the license and selecting evaluation). If this is not the case you may contact a VMware representative or your VMware reseller and ask them for evaluation licenses.
Once you finished evaluation, removed the hosts from the vCenter Server and disabled all advanced features which are not part of the free edition, you should be able to re-assign the free license.
André