are you wanting the whole mag tube or just a extension? most 500s only have a end screw, so you'd have to replace the whole mag tube.
Do you have a end cap or end screw set up?
If its a screw type , like in the pic of the combo,you my friend are SOL,you'll have to replace the whole mag tube and get a new barrel,to get the extra rounds you want, they do make kits for this,"MOSSBERG 500 HIGH CAPACITY SHOTGUN KIT", not sure if you can get them up here. saw them on the Brownells web site,they run about $150 U.S.
How long of barrel do you have on your 500 right now?
Sweet, thats the spirit, now you just have to decide if you want the big one , 20" barrel 8+1rd and a Bayonet lug, which is cool or the 18.5" barreled one, the shorter one you can get +1 or +2 mag ext for like 6,7+1rd.