Never heard the term jackstick before, but you mean a 3.5mm audio jack, right? In that case you can build a volume box which also has a stereo audio jacks input and output, and connect it to the amplifier using a standard audio cable. This is how I added per-channel volume controls to my box. That ...

Building my own box based on the schematic on page 12. I found a card that has jackstick and bluetooth connections. It seemed decent. Issue is, I just realized the pots go on the input side of things rather than the output, and that's not possible unless I desolder the jackstick and somehow add them...