What you could consider doing is upload the files into 2 separate tables in Access (DoCmd.TransferSpreadsheet) will work then you can run some SQL code to update the comments. That would work something like this:
sql = "UPDATE [XIAP Stub dataset New] INNER JOIN [Underwriter Codes] ON [XIAP Stub dataset New].[Uwr ID Code] = [Underwriter Codes].[System Code]"
sql = sql & "SET [XIAP Stub dataset New].[Uwr ID Code] = [Underwriter Codes]![XIAP Code];" ' The table to update 'the field and content to add