Yes, you can add more fields (new questions) to repeats. The safest way to do this is to modify the schema of the feature layer in AGO/Portal or via ArcGIS Pro/Desktop or via the REST endpoint. This allows you to make the required changes to the related layer (add a new field) safely in the feature service. You can then add that field to the xlsx form in Connect by adding a new question in the begin repeat section of that related table, and give it the same name in the name column as the new field name you added. When re-publishing it will find that this field already exists and publish successfully.
Currently there is a known limitation with re-publishing the survey if you try to add the new field in the survey xslxform directly, without adding it to feature layer first. Connect may fail to publish and hang in some cases.
If you are only adding a new field to the feature layer/table, then no, data should not be deleted from the existing fields in the same layer/table. However, it is always a good idea to back up your feature service (export to fgdb or create a copy), before modifying the schema via any of the above suggested ways.