Hello Axel,
Thanks for reaching out to the Business Analyst team. I would like to inform you that we are considering this workflow for a future release but we don't have a specific timeline yet. In the meantime, I hope the following workaround works for you:
1. Go to Add Data > Import File > Browse and Import your file
2. Select the Geographic boundaries option > match your boundary fields (eg. Zip codes)
3. Click Next > Save the layer > click Organize Sites.
Note: You will see the imported layer is saved under Geographies (Sites) now. This layer is saved in your project feature service. Notice the name of my project is "Thematic Demo".
4. Un-check the check box next to the layer to remove it from the map.
5. Go to Add Data > Custom Data Setup > Start Setup
6. Choose Layer > Browse for a layer > Click on My Groups tab and sort by Recent.
Note: You'll see your project feature services listed here. And you can see "Thematic Demo Layers" is the feature services that contains the imported file.
7. Hover over that item and click Add > select the imported layer.
8. Click Setup Custom Data > Drag and drop the variables you want to map to the right.
Note: You can learn more about Custom Data here: Custom Data Setup—Business Analyst Web App | ArcGIS
9. Click Save and you will see the following options > Click Color-Coded Maps.
10. Select the desired variable to and you can create a thematic map from your imported layers as follows:
Note: You can use your custom data in any supported workflows by browsing to it in the data browser (My Data).
I hope this is helpful,
Best regards,
Fasil T.
Business Analyst Team