3704642 - How to mass edit SNC Names using custom variable part on SNC1 / RSUSR300

When trying to update SNC Names massively via transaction SNC1 / report RSUSR300, the only standard option for the variable part is either User Name or Alias.
There's a need to use some other attribute as variable part, such as the Email maintained in transaction SU01. 

"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."

• ABAP platform
• SAP NetWeaver

SNC Name, Email as variable, e-mail as variable, custom variable, custom SNC Name, SNC1, RSUSR300, BAdI, set external security name for all users , KBA , BC-SEC-USR-ADM , Users and Authorization administration , BC-SEC-SNC-CFG , SNC Configuration for ABAP , How To