Hello
If you ONLY want to switch between the original system and piezo at output it is straightworfard - add the switch - wire preamp output to one side and humbucker on one side and the center to output jack.
If you wish to have the guitar electronics affect the humbucker, too - you probably must attenuate the output of humbucker with series resistor, or series resistor plus drain resistor. This is fairly easy and the benefit is that the humbucker now "sees" higher impedance thus being brighter.
For the latter the connection is humbucker to series resistor - series resistor to one side of the switch - shunt resistor from the switch to ground - piezo on other end. That´s it. Unfortunately I can not tell you the values of the resistors without trying it with my Songbird - and I have no intention to do so - but since you have that nice assembly in the soundhole, which is where I suppose you plan to mount the switch, there is a fairly easy shortcut. All you have to lose are few bucks, little time and two holes in your new soundhole cover. ( If you drill elsewhere - then that´s how it will be )
But anyway - take a volume pot of 250k - 500k - 1M - anything round there will do. Wire the humbucker to top of that - other end to ground - the center viper to one end of the switch. Piezo to other end - center to preamp input, where piezo was originally. My Songbird has a block with screw contacts, so all will be reversible. Just make notes how it was before you start pulling cables out. Be sure to use a two-way switch. So one or the other. Parallel will not work, since the output impedance of the humbucker is low enough to load your piezo so it will not sound good. Nothing will go kaputt, but it is no good.
Have fun.
p.s. Adorshki was writing faster than I was - but the input impedance is no issue in this case - the higher te input impedance, the merrier. But the humbucker will produce way more LEVEL than piezo - hence the attenuator.