Hi
A sequential gearbox is possible, but it would require a major rework of the internals. Most sequential gearboxes have ALL of the movable elements on the same shaft. This is not the case with the Fiat transaxle. Then you still have to accommodate reverse gear.
Bacci Romano currently make a sequential gearbox for the Fiat 500. The cost is over 12,000 Euro !! It is not substantial enough for use with in a 1000TC automobile in my opinion.