I mean the area of the filter furthest from the tank at the back of the filter that's full of water when the filter is running. There should be enough room between there and the cartridges.
Here's a rough drawing:
The red is the filter intake tube, the blue is the filter cartridge, and the pink/purple is the biowheel. The brown ellipse is where you could stick the media bag full of peat.
Even if you position the bag of peat between the filter cartridge(s) and the biowheel, the tannins will still be removed from the water just as fast as they are added as long as any of your filter cartridges have carbon in them. Again, I'm not sure exactly how much the removal of the discoloration will negatively impact the peat's ability to lower pH, but I imagine you'd just be better off with no carbon.