Apr 24, 2020 3:35 PM
You are able to manually add multiple email addresses to contacts, so if a new contact is being created that has the email as secondary Hubspot will indicate that the email is being associated with a contact already.
This article explains more about it: Add multiple email addresses to a contact
Currently it is not possible to create a custom property that will accept email. We have an idea in our ideas forum where our customer is also requesting a similar feature, my recommendation is if you could please leave your comment in that idea so you can receive any upcoming updates about it.
I hope this helps,