You should be able to apply classifications under your Customer ID eVar through the classification importer... You would need a classification for each variant, since a classification can only hold a single value, but maybe that is all you need? If you are just trying to identify "in-store purchaser"
So let's say you had these customers:
1234
3566
348
Let's say you identify 1234 as such a person, you could flag you classification as "true"
But the problem here would be, this is basically an all time mapping, as opposed to each individual in-store purchase....
You might be better off looking at importing real data, about those in-store purchases, using Data Sources.
While this example uses Importing GA, it could be modified to use any custom data source.
https://www.fullstackanalyst.io/blog/adobe-analytics/import-google-analytics-data-into-adobe-analyti...
This way, assuming you have the information about the in store purchase, you could have revenue, product info, quantities, and importantly timestamps for use with pulling reports within whatever time frame you need to review.