How to add clients who already have an account?

The client, who you want to add, needs to write a mail to with the following text: 

Please assign my account [e-mail address of the customer account] to the account of my supervising agency [e-mail address of the agency account].
I understand that this gives the agency complete access to my account and, in particular, all admin rights. I also understand that the agency receives access to the personal data in my account. We clarify this legal relationship with each other.

It is important that this e-mail is sent from the e-mail address of the account holder, otherwise we can not make an assignment.