Hi!
Peer manual specification is not possible now, i've created a ticket for that, but there's no exact schedule when this could be implemented. https://track.radensolutions.com/issue/NX-1973
As for adding inventory information to nodes that have no agent and no snmp - currently inventory is populated only by server and is read-only. As developer resources are limited, some manual way could be implemented only if someone requests paid developmet (or, possibly contributes a working code for this).
Currently it's possible to store such information in custom attributes, but there's no way to display that information in the GUI.
Or, alternatively it's possible to create a push dci on the node and use a script object tool that have input field and will update the push DCI.