Agreed, dumping the entire secondary datasource's xml into a single element within your main datasource would not be best. I think you are on the right path but just need to tweek it. In your main data source add a repeating group(s) that match the schema of your secondary datasource. Then as items are checked in your secondary datasource repeating group, move the data into your main datasource.
Jason
http://www.k2distillery.com/