I have heard of plenty of ‘off-the-shelf’ GTR sequential boxes – but not of a sequential conversion kit (not for the GTR that is). I’d say it would be quite expensive considering a new sequential box is upwards of $20k. You’d probably need at least a new gearkit/selectors, shifter mechanism, levers, etc. maybe even a new clutch, to modify a standard box if it is even possible. Then you’d probably want to add hydrualic actuation, paddle shift, etc so you can be just like Schumacher. But if you have the $$$...