Hi @franc,
As a supplement of what @stephanealoo has advised, would you also mind having a look at the workaround that has been discussed earlier. It should solve your issue:
Importing xls data into KoBoToolbox Data Management
@sareep, a while back have created a method for converting XLS data into XML with a Python script so that they can be submitted via the API. This requires an existing deployed form to be in your account. Everything is documented in this repository: GitHub - kobotoolbox/xls-import: ⚠ THIS IS NOT A SUPPORTED REPOSITORY. It is here with the hope that it's helpful for your own efforts.. I hope this works for you.
Have a great day!
