Using the same email more than once

Can the software be changed to accept the use of the same email address on more than one license/ company? 

It is far too restrictive that an email address can only be used once instead of across more than one license - I have 5 licenses under my account, 2 of the licenses belong to sister companies, therefore I only have one email address to cover both companies. 

I have also been unable to add the company's accountancy company as their email address is already linked to other licenses. 

Can something be done to make this less restrictive?