Details, man , don't just leave us hanging. lol
You'll need your wife's hairdryer, a 7 or 8" round file, a 16mm socket (actually I don't remember exact size, it's gotta fit tightly into the mag tube) a long socket extension and a hammer.
Disassemble the shotgun and unscrew the mag tube from the receiver. You will need to heat the area where the mag tube meets the receiver for a few minutes and then it will unscrew easily by hand.
Keep the red aluminum follower, mag spring but discard the red aluminum end cap that sits against the 3 ribs in the mag tube.
Use the file to remove as much of the 3 ribs as you can. Note that to get the file into the mag tube properly you will have to remove a bit of material from the end of the mag tube where the cap screws on. For some reason they've tapered it here. I did this on a bench grinder to save time but you could do this with the file as well. Clean up the threads because the cap has to screw back on. Cold blue any exposed steel to prevent corrosion.
Once you've got most of the ribs removed, drop your socket down the mag tube and see if it slides freely over the ribs. If it doesn't, go back to filing. Once you've got it so the socket kind of passes the ribs, use the long socket extension and tap it a few times with the hammer. It should round out any tiny imperfections. Don't go crazy hammering, you will end up with a socket jammed in your mag tube. Do 99% of the work with the file.
Once the socket passes freely, drop the follower in and make sure it also passes freely. You don't want to half ass it as you will end up with an unreliable gun.
Blue what you can. Clean out the mag tube and make sure it's free of filings. My final wipe through I put a tiny bit of oil on the rag.
Reinstall the spring, follower, mag tube into the receiver and check the function with snap caps. Remember to heat the receiver back up before you screw in the mag tube because there's still thread locker in there and you want it soft before you re-install.
It should now fit 7x 2 3/4" in the tube + 2 for a total of 9 rounds.
Note that I don't have 3" shells to test, it's entirely possible this is illegal.
If you can fit a 6th 3" shell the above is against the law.