I would recommend against as you would not be able to fully populate the hardware health/ list inventory information. I would recommend a discovery even if you limited it to just those IP's you expected to come online at some point so it could fully discovery the interfaces/volumes, etc
Just playing around though - I took an ICMP node and did a Database Manager (be afraid, be very afraid ;-) update script that not only switched the node to SNMP but because mine is up it returns ICMP and Orion thinks it's still just fine.
update Nodes set objectsubtype='SNMP',community='your_community_string',snmpversion='2'
where nodeid = your_node_id;
*****If you mistakenly get the where clause wrong you could overwrite all your nodes - beware.
