I have a related table (competitors) that allows us to choose multiple values on the sales form. I need to import existing data. For other 1:1 related fields, it's easy - I just import the ID of that related record. For this one, I don't see how to import multiple values in the import. Any ideas?
In that case you would need to import into that Join table and provide the Record ID# of the sales form record as [Related sales form] and also the Record ID of the Competitors table as the [Related Competitor], if the key field of that Competitors table is the [Record ID#].
If that is not the setup of your relationships, post back with what tables you have and how they are related.
"Or, do I have to do this by doing an import into the middle join table and not via the sales import?"
Yes, you need to import into that middle Join table and include the two field typically called [Related sales form] and [Related Competitor]. These are typically numeric fields corresponding to the Record ID# of each of the two Parent Records.