That would need you to know how to use newly inserted IDs in consecutive queries at least. Not to speak about how easily one can mess up with all metadata by doing things outside of phpBB. Consider using
function user_add() in /includes/functions_user.php instead - with your own script it'd be also very easy to adjust timeout thresholds and run loops over "several thousands". You can look up on how /includes/ucp/ucp_register.php deals with creating accounts.
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