Allow other unique identifiers for CS -> PX

For data privacy reasons the users records that we load to PX are anonymous, i.e they don’t have their name or e-mail included. 

In CS, when we bring the PX data in via the connector, we match the users back to their name/email using the PX ID as the unique identifier. 

However we’re currently unable to do the same in reverse - we want to run an in-app engagement triggered by JO in CS but the target audience would be based on matching by e-mail address (which doesn’t exist in PX). 

Please can you allow other unique identifiers (our use case would be PX ID) to be used between the platforms?