When you add a geopoint question to your XLSForm, Survey123 Connect expects your target layer to have a geometry field. In your case, the target layer is a table. If somehow you can add a geometry field to your table, Survey123 Connect will not complain. For example, if you published your feature service from ArcGIS Pro, you should be able to overwrite your service.
Since it looks like you will need to recreate you table anyways, you could do the following:
- Use the Save as option in Survey123 Connect to make a copy of your survey design
- In the new form, go into the settings worksheet and clear the submission_url an form_id cells
- Add your geopoint question and make other changes to your design
- Publish your survey. This will create a new folder in your account and a new feature service
- Next append data into you empty new feature services using the old layer and table as a source