Hello @PaulGerbeaud,
If you published the survey from Survey123 Connect if you add the nested repeat and republish the existing feature service will be deleted and a new one will be created.
I would suggest downloading the feature service as a file geodatabase, using ArcGIS desktop adding the new table/layer in the file geodatabase and manually create the relationship. Once everything is set up you can overwrite the existing feature service with the updates.
This way you will be able to keep existing data that's present in the feature service.
Once the new related table exists in the feature service you can add the repeat in your XLSForm and since the schema will match Survey123 will be able to publish without an issue.
Thank you,
Zach
Thank you,
Zach