Emulator based Driving cabs take a lot of work to get them working well.
I’m not familiar with the makeup of the NFS cabs, is it Linux based?… can you upload some photos of the internals?
From the info given, you have a couple of options I’d say;
1. Adapt the PC that’s in there already to set up a dual boot system, one for the original NFS stuff and one to boot into the Emulator based OS. The spec of this PC might not have enough grunt to play the more demanding emulated stuff though.
2. Setup another PC with all your Emulated stuff on. You would need to switch cables from the NFS PC to this PC when you want to swap between the two systems. If all your controls are connected to the Immersion board, then it’s simply a matter of switching the USB cable from this board between the PC’s to interface the controls. Then the controls need to be configured. If there is another I/O board connecting some of the controls then this would also need to be interfaced to the new PC if compatible.
Which ever way you go, each emulated game will need to be configured and tweaked individually to get the game playing correctly, that in itself takes a lot of time.