Since OP had this issue in 2016, it was probably resolved. But in the last couple of weeks since HubSpot has updated their imports tool, CRM no longer has a way to import an opt out list.
Also, the properties for opt-out (both marketing and one-to-one emails) are read only, so you can’t update them with a separate column in a CSV.
This is a major issue, HubSpot, please address it!