As an update on this issue - the only workaround that we have found so far is to develop a custom webapp that uses the hubspot API to take contacts from a google spreadsheet and submit them to a form on a landing page specially created for each upload, so we can attach contacts to a campaign.
This was a lot of work for an issue that seems like it’s very common so should be handled by the product!!!
Cheers
PS: If you’re interested in this solution, let me know!