The current release version of Archive To Go is 1.2 (not 2.1).
Is the “external email archive system” a native/personal GroupWise archive? If so, it may be necessary to use the FID Editor to change the FID in this archive (to match that of an account in a GroupWise post office). Then it should be possible for Archive To Go to export the native archive – with or without the main (‘live’) account.