It is not possible to show or modify the data in the configured UDM property (umc/self-service/passwordreset/external/udm_property). In set_contact_data() it is hard coded for email and sms only. We should at least provide another extAttrib for a default solution that works out of the box. A generalized solution for "external" would have to take precautions regarding the type of attribute (possibly multiple values) resulting in different UI widgets and load/store code.
*** This bug has been marked as a duplicate of bug 39952 ***