Enable forms to access data from an external source

We don't have immediate plans to connect directly from clients to arbitrary data sources though it's something that we will likely consider in the future.

If the data you want to feed back into follow-up forms is from ODK submissions, Entities should be a good match.

If the data exists in some external source of truth, you can consider using an external process to keep a Central entity list synchronized with it. For example, see the discussion here.