i'm not exactly sure what you mean either.
is it possible to make your car both an automatic and a manual? NO. tiptronic accomplishes a slight version of this but it is nothing like a true manual...push buttons do NOT make for a single clutch & stick. (an automatic 4-speed can be said to have a clutch for each gear..althought their obviously not called clutches)
No, actually, they are called Clutch disks or clutch rings, like you said, each gear has it's own clutch.
This is why you rarely have to replace the clutch rings on an automatic, since each gear has it's own friction material/clutch band, they don't burn out nearly as quick as a manual, which uses the same clutch to grab all the gears; it gets more of a workout, and are vartiable in pressure because they are operated by the driver, thus they burn out quicker.
In an auto tranny, the clutches are engaged by fluid pressure and solenoids, much more consistent in operation; very reliable too.
The GTS Sprtshift is a decent transmission, but the lack of a 5th gear and a low stall speed are it's achillies heel. One is fixable (better torque convertor to raise stall speed, valve body calibration to shorten up gear changes) the other is not (Another gear cannot be added, of course, smaller gearing IS an option for those creative enough)
The closest thing I can think of to something that's both automatic and manual is a sequential transmission (Like the one from the newer MR2s)
This is the way to go if you're serious. Some people may talk trash about the sequentials on the MR2, for good reason; it's a piece of junk when compared to something like the Quaife 6-speed sequential.
The Quaife will work great, can have any gear ratios you want, and they have adapter housings for any application you can think of (including our cars) but they're not cheap... Prepare to spend over $15K on one of those bad boys! they are primarily used in professional racing (touring cars mostly) but are worth every penny, and are about as bullet-proof as you can get!
Of course, we haven't even touched on some of the other options out there, such as air-shifters (like those used in drag racing) or the new X-Mission FWD import racing transmissions. none of these options would be cheap...
As for the MR2 sequential gearbox, it may work fine with the GT, but if you have a 2ZZ engine, it may not hold up (as far as I know, nobody has run one sucessfully with a 2ZZ engine yet)
Oh yea, the cadillac tranny you were talking about is an tiptronic automatic (sililar to the GTS auto) Cadillac Nortstars only have one tranny available, andf that's the tiptronic automatic (they don't come with manual trannys at all)