I've already imported deals and companies into HubSpot. But I'd like to add new companies for those existing deals as a primary company. For now, the existing companies in HubSpot exist as primary companies for the deals.
You can add a column in the spreadsheet called Association Label and when mapping the properties before import map this column as Association Label.
The column for Association Label on your spreadsheet should contain the word Primary on every row.
Should you have any questions regarding setting up your import file or would like to get on a screen share, I would recommend connecting with HubSpot Technical Support, as Support is included in your subscription. They will be able to provide real-time assistance for this matter, where you are able to share screenshots and further information about this.