If you remove the screw-on cap on the end of the magazine tube to remove the barrel, you will notice a wooden plug at that forward end of the magazine. Removing that will provide the magazine capacity you desire. It will also render your gun illegal to hunt migratory waterfowl, as others have mentioned. The green plug you can see through the loading port, at the aft end of the magazine, has nothing to do with magazine capacity and needs to remain as-is to ensure reliable feeding.
The most predictable thing about steel shot is its unpredictability. You simply have to pattern your gun with the specific load you will be shooting. Some guns will throw full choke patterns with improved cylinder chokes installed with certain ammo. Some guns will actually need a full choke to throw full choke patterns with certain ammo.
Have you considered other non-toxic shot? There are far better choices than steel these days. Hell, I even found one that is safe to shoot "bare" in muzzleloaders. It's made by Ballistic Products; they call it their "ITX" shot.
ITX Original-10-ballisticproducts.com
I'll be using it this season in my side by side 10 gauge double muzzle loader. I'm not sure if it's available in loaded ammo, but there is plenty of bismuth, tungsten, and others available that do far better than steel.