You can buy a harness for ECU nitrous contol, but all it is doing is doing for activation is grounding either the mode up or mode down button input to the ECU (same as pushing the button yourself). You can do it with a switch or with a different momentary button to ground if you like. You can have a three position switch to have "auto", "manual" and "off" if you like.
Then for the output you simply use the two wires from the PAIR to activate a relay, then use the relay to supply power (or ground) to your solenoid.
If you buy the harness it is mostly plug and play, but if you want things set up a little differently then you are doing your own work anyway. $10 gets you a switch and relay.