How to import subform data - SOLUTION

To all trying to import subform data, I might have a solution for you.

First of all, for this solution to work, the subform data needs to be an independent form (not ad hoc created on the main form). Furthermore, this approach uses Excel sheets - it might not work using CSV/TSV.

If this is true, then follow these steps:

Import the subform records
Then export these records once more including their ID
Now prepare an import file for the main form that needs to contain the subform records
Within this import file, add a column (any name) and fill it with comma separated values of the ID's that you exported in step 2.
The ID's should be enclosed in " (quotes) like this: "18000299204099","39809803021393","99809803309"
Save the file and prepare an import for the main form records
Make sure to include subform data and select the added column with the subform ID's for the subform field of the main form
Start the import...

After this, the main forms imported should have the subform records related just as you would when creating them manually.

At least it worked for me ;-)