Hello RTM RTM,
Depending on the format of your customer records it would need to be appended to the Feature Service associated with your Survey123 survey.
One option would be to use the append tool in ArcGIS Online/Enterprise Portal: Manage hosted feature layers—ArcGIS Online Help | Documentation
Although the more pragmatic workflow would be to use the append Geoprocessing tool in ArcGIS Pro: Append (Data Management)—ArcGIS Pro | Documentation
If your customer data is a little more complicated with attachments and related records the following video may be helpful with the append process: https://www.youtube.com/watch?v=S5V6MIrAKCk&t=1s