Official comment
The shortcuts you describe, for Link Export, are to load songs to linked hardware players, not the virtual players. Oddly enough, there are no shortcuts to load tracks to the virtual players under EXPORT mode.
You're actually better to try using PERFORMANCE mode, which is where the magic happens with a controller. EXPORT mode is really more for music preparation / organization, and previewing songs. PERFORMANCE mode certainly has shortcuts allowing you to use the keyboard to load tracks to decks -- but a hardware controller also has those features, negating the need to assign shortcuts.