Great question... I need that too.
As you got no answers until now, I assume this is not possible. There is no command for this is TABCMD.
One "sloppy" way you could automate this is via AutoHotKey. This freeware enables you to create macros for Windows. You can create a script with it that opens up Tableau Desktop, load the main TDE file, append the latest file to it, and closes Tableau Desktop at the end.
The application is very robust, and you can create some serious logic and controls with it.
If you indeed take the time to write such a script for AHK, please share! It would help a lot of people.
Also, please vote up for this functionality in next versions of Tableau: