I would use either the ST1 or ST2 stereo inputs on the back of the mixer.
You would need RCA to 1/4 cables for that.
ST1 and ST2 are dedicated stereo inputs on one fader
on the QU32 surface they are on the second layer.
You can build a custom layer and include the channel on that.
If ST1 and ST2 are already in use you can as mention link a pair of input channels together to operate as one stereo pair, pan the channels accordingly for stereo.
You can only link an odd number channel to the next even number channel.
The ST3 input on the top surface of the mixer would work but if you are using the
the USB for some playback ST3 is dedicated to the USB playback input and more so
I would not want a somewhat fragile 1/8 inch plug sticking up on the top mixer just
waiting to get hit or pulled on and broken off.
I like get away from using any 1/8 connector as fast as possible or not at all when connecting anything in a sound system.