It would be nice if there was a way to take an existing node (or nodes), and import it (or mass import them) into an existing provisioning group without losing any historical data.

Something that would essentially do (for a single node):
psql> UPDATE node set foreignsource = 'ProvisioningGroup', foreignid = 'uniqueid' where nodeid = ${nodeid};

Then take the existing interfaces, services and categories, and create the provisioning group entry, and then synchronize the group.