Associate webform submissions with the user created
I would recommend checking out the available hooks provided by the Webform module: http://cgit.drupalcode.org/webform/tree/webform.api.php

It looks like hook_webform_submission_insert() will be your best bet, with that you should be able to not only programmatically create the user at that point, but then update the submission with the newly created user id.

function my_module_webform_submission_insert($node, $submission) {
  // Create new user.
  // Retrieve new user id.
  // Update webform submission with new user id.
}