Can I implement a Save Plan for a Savable Data Page that loaded its data using "Aggregate Sources"?
Yes. How the Data Page is loaded is independent of how the Data Page is saved.
The Data Page can - this way - become a key vehicle for progressively migrating data from a legacy system of record ('L') to the strategic system of record ('S'). The Data Page's Data Sources (for retrieval) could Aggregate across 'L' and 'S', with Post Load Processing filtering out any records from 'L' that are already in 'S'.
If that Data Page's Save Plan was to only write to 'S', then 'S' would progressively attract the latest data touched by the Pega application, without a discrete data migration exercise.