[Andreas]

Hmm, I think the easiest way would be to create a script to that uses your existing account-data to create vB-Accounts.

Moreless, for each user you will have to create a new record in table user, then call $DB_site->insert_id() to get the userid and afterwards create the accompanying records in the usertextfield and userfield table.

When finished you should also call build_user_statistics(); from functions_databuild.php to update some caches.