How do I add to a rule without the UI?

Jochen Wendebaum’s comment on https://www.drupal.org/node/1593318 exactly addresses the problem I am trying to solve – set a condition “if a drupal user does not exist” (for an email adress from civicrm)… then create a drupal user.
However, I’ve no idea what to do with that snippet – how to enter php directly for a rule – how are they stored?

Alternatively, is there a way to create a “if drupal user with this email does not already exist” condition via the rules UI?

Richard

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