I currently have a vCenter and ESxi in version 5.1.
I just deployed a new vCenter and ESxi in versions 7.0
One option would be to export those VMs to ova and then import them to the new vcenter.
But I would like to know if it is feasible to add those vCenter 5.1 nodes to the new vCenter 7.0 cluster and then with a vmotion move those VMs to the new ESxi 7.0
It is not possible to connect esxi5.1 to vCenter 7.0. vCenter 7.0 supports esxi 6.5 and higher.
You can try to mount datastore to esxi.7.0 (if u have one in vCenter7.0) and remove vm from vSphere 5.1 and register this in vsphere 7.0 (vCenter 7.0) browsing datastore.
According to this tool: https://interopmatrix.vmware.com/Interoperability
vCenter Server 7.x cannot manage an ESXi 5.1 host.
Can you give the new hosts access to the old datastores, even temporarily?