Hi @athoman and welcome, it’s a pleasure to have you here! 
Thank you for asking the HubSpot Community and sharing this detailed example!
I love you asked this question because imports can be tricky at times.
I have checked for you and I can see on this article “Import records and activities” (there is also an example of how to set it up) that if you’re importing and associating objects, to import association labels (Professional and Enterprise only): you can click the dropdown menu in the Import as column, then select Association label. Importing a new association label will not overwrite an existing association label. The imported label will be added to the record as an additional association label.
I’d like to share also this article “Requirements to associate records” which can help you set up the import files correctly.
If you are getting errors while importing, you’ll want to have a look at this: “Review and troubleshoot import errors”.
I’d love to put you in touch with some of our Top Experts who have great experience and insights when it comes to importing data: Hi @danmoyle, @GeorgeBThomas and @AdamLPW do you have suggestions to help @athoman, please?
And please @athoman, let me know if there is anything else I can help with.
I’ll be delighted to do so! 
Thank you so much 
and have a wonderful day!
Best,
Bérangère
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