Is it possible to link the mono mixes as stereo pairs in the Qu SB?
Nope
If you need more stereo mixes remember that you can switch the groups
to be stereo auxes/mixes.
The catch there is that you will need a stage box to access the additional outputs.
You can actually get one more stereo mix directly out of the QU SB from switching a group to a mix and then sending that to the matrix and using the matrix 1 - 2 outputs.