Short answer
HubSpot's KB: add extra emails on the record, by import, or in the Android app. Set the main address as primary, because HubSpot tools reference it first and Salesforce syncs only the primary. Imports can add emails (separate several with semicolons) but can't set the primary. Bounces and unsubscribes belong to an address, not a contact.
1. The record
A contact record in my demo portal.

2. Merges
Merging contacts keeps both addresses. See merging records.