First, I was unaware that the printing/Exporting from Outlook of a single email message was being created into a PDF Portfolio. The file extension does not reveal that it is anything different from a PDF Document.
When dragging and dropping that file/Portfolio to append a document in Acrobat DC, the Portfolio is only shown as an advertisement to Get Adobe Reader. There is no access to the document(s) contained and the append operation fails.
Please utilize a distinct/different file extension to indicate PDF Portfolios.
Allow the Add/Append to either Extract the pages of the Portfolio for an Append Process or enable the viewing of the Portfolio section from within the Adobe Pro/Reader.