You can retain your OEM cable reel with the NRG quick release hub, from there, you tap into the yellow connectors (there are two of them) for horn signal (should be +12V, contact to ground and the horn beeps), as well as for your cruise control buttons - you're not going to use the SRS wiring, although you jump two of the pins together, to eliminate the SRS light.
For the horn button, one side goes to the +12V horn signal on the cable reel connector, the other side of the button is connected to the hub, which is connected to ground via the steering rack. You can test this, before bolting everything together, by jumping horn signal to ground (just hit the wire against the hub and it should beep), the car does not need to be turned on for this to work.
There is a writeup on
http://www.nsxbuilder.com, but he is using a momo hub, which does not allow for the cable reel - he does provide a picture of the cable reel wiring, although it is the opposite end, as he removed his cable reel assembly (while you're going to keep it, for horn and cruise control functionality).
You can also trace the connectors using the nsx service manual.