No, unfortunately not until now. I use the workaround....and drive only automatic :-)
If I am understanding it, your trans will not hold in the right quadrant of the His & Hers shifter, 1,2,3, and simply acts as any transmission in Drive?
Is the His & Hers original to your car? If so.......
Someone may have swapped out the original TH-400. Do you know it is the original one to your car? The GTO TH-400 is set up within the valve body to either act as a "normal" automatic in Drive or be shifted manually in the 1-2-3 right quadrant. In the right side, the transmission did not shift until you shifted it. If the transmission was swapped out, rebuilt with a replacement valve body, or the valve body itself replaced for some reason, you would not get the 1-2-3 manual shifting of the TH-400 as it is the valve body which controls this.
You can either get an aftermarket valve body that would give you the control of the trans like factory OR buy and aftermarket kit like TransGo like this one: TransGo THM 400, 3L80: 400-1&2
They also have another upgrade if you want to go even more race, but the one above is what you would want and it will give you snappy fast shifts. It will act as an automatic in Drive, or manual shift 1-2, 2-3, when you want to, or keep it in the gear of your choice 'til you blow the engine 'cause it won't shift until you do. Had a TransGo shift kit installed during the rebuild of my brother's Mopar 360 w/904 automatic and it does just as advertised. It'll catch gears - just like my '68 GTO with its factory His & Hers used to do.
If you are handy with tools and feel comfortable working on the trans, you can do it yourself. Otherwise, might be best to just take it to a trans shop and let them do it. I have done the B&M kit in TH-400's in my youth without much problem, but had a shop do the TransGo kit as they were doing a full rebuild of the trans.
Also, here is a little more info on the His & Hers that might help and add some clarification:
phscollectorcarworld: Hot Options: Hurst Dual Gate "His n Her" Shifter
Hope that helps.