There was a DIY done a while back that involved the whole 7AT shift booth being converted to look like a 6MT, the op there used a Password JDM shift nob:
Password:JDM Balanced Shift Knob (Nissan 370Z) | Password JDM
I dont know if it worked out but there was an alternative method used as well, you can probably find more info here:
DIY OEM Shift Boot Install on Automatic