Yes, you can! The Supermetrics HubSpot connector supports custom properties for "Contacts", "Companies", and "Deals". These fields should become available when the latest import for those completes and they will be compatible with fields within their own types.
If the custom properties are not appearing for you, please check to make sure the last import completed recently (after October 24th, 2019) for "Contacts", "Companies", and/or "Deals" with the Monitoring Tools.
If the import completed but you still do not see them in your Supermetrics product, please follow the directions in Forcing a Refresh for Account Data (making sure to do the final step to re-authenticate) and then they should become available.
If you have any issues or feedback about this feature, please submit a support request so that we can assist.