Adding a repeat to an existing survey is very tricky, I don't think S123 Connect can do it while maintaining your data (I could be wrong though). It's been a while since I've done this, but I think you could do it a few different ways:
Method 1
- Export and download your existing service's schema to a FGDB
- Add a new table & relationship class for the repeat
- Zip the FGDB and publish as a new service, modify the settings to match your old service (views, sharing, editing settings, etc.)
- Republish the form to point to the new service
This has some downsides, like if you have webmaps/apps that use your S123 service they'll have to be modified to point to the new service. Additionally, FGDB exports tend to flake out if you have more than (approximately) 1.5GB worth of data - feature + attachments - in the export.
Method 2
- Open the administrative REST API of the service
- Use the Add to Definition endpoint to add a new table
- Use the same endpoint to define a relationship between the parent and the child
- Modify the form to include the repeat and republish
This is "cleaner" but it's really easy to completely screw up your service this way. You really want to practice this on some garbage data before trying it against anything that matters.
Method 3
- Publish your form to a new service entirely, that includes the repeat
- Use the Python API to script up an ETL process to drag your old records and attachments over to the new service
The downside here is that it can be fair bit of work, especially if you're not handy with Python. Also, repointing webmaps/apps and such.
