I spoke with Mark Bowler last week. Bowler Transmissions has two products that make a true auto six speed manumatic a reality using an existing four speed auto and a gear vendors overdrive:
1) Modified Valve Body: Provides full engine braking when the gear selector is in the "3" position. This product is still undergoing transmission durability testing. But you can "hear" it in action in this video.
Engine Braking Valve Body Mods
2) GV Interface Electronics: Mark has electronics that will actuate both the transmission shift solenoids AND the gear splitter from the same switching mechanism such as my paddle shifter.
This is technically an eight speed transmission when the gear selector is in the OD position. However there is no engine braking in this mode.