How do I modify this schematic to be input/output?
What you need is high impedance, in other words, the current coming from J3 doesn't leak into J2. But you want the voltage to be copied. That asks for an operational amplifier in follower (aka buffer) mode.

I copied your schematic so you would feel most familiar with it.



Here's the link if you want to.. I don't know, mess around. 

Nearly any op-amp will do the job. If you want to be sure that it works for you, then make sure that the bandwidth of the op-amp is at least 100kHz for some margin (audible max frequency is usually around 22kHz). I'm not 100% sure if the voltage will be flying all over the place, but if you are aware that it does, then you might consider using a rail-to-rail to make sure that the op-amp copies the voltage from J3 for all voltages. (LM358 for an example can output 0 to VDD-1.5V, so if you feed an LM358 with 5V then its output will only be from 0V to 3.5V)

Keep in mind that whatever op-amp you use to do this, its output will always be within what you're feeding the op-amp with. So if you feed the op-amp with -9V and 9V then its output will be able to go near -9V and near 9V. In other words, if your audio is centered around 0V, aka GND, then you shouldn't use GND as your negative voltage supply for the op-amp, because then the op-amp won't be able to make the negative voltages that the audio is making. It will be clipping the audio and it will sound nasty. 

My final answer/advice is this: Follow your own schematic, you will lose some sound and introduce some noise, you might not hear the difference and everything will be nice and dandy. Or you will hear that the sound is about half because J3 is trying to drive two systems. Either way, if it doesn't work, then I'd do my schematic and make sure that the op-amp can output all the voltages you feed it. So if the voltage will dip down to -3V, then I'd make sure to feed the op-amp with at least -3V so it can make that voltage at its output. So this answer greatly depends on what your inputs voltage actually is like.