Hi there,
You can always query the PrivateMembersData collection and insert a specific field into the new database.
Depending on how you’re registering new members you might be able to use data hooks to automatically insert records into the new collection after it has been inserted into your PrivateMembersData collection.
As alternative, you can also export your data from the members’ collection by field and import into the new collection, however, this is less practical.
Hope this helped.
Best regards,