iSupport: newly created user is not assigned to account automatically, bug 13504836 could not be ver

I'm implementing iSupport at the moment and had some issues, which were resolved by assigning the user to an account.

Self-registration provides an option to fill in a register-id for the company, but it's account is not linked to the newly created user afterwards. The flow is as follows:

1. guest registers through ibuhpage.jsp
2. guest chooses "Register as a user of an existing company"
3. guest enters "Registry ID" (which is party_number from hz_parties by the way)
4. guest chooses a username
5. new user is created, an approval is sent to sysadmin
6. approval is accepted in CRM HTML Administrator
7. sysadmin manually assigns user to the account in CRM HTML Administrator