You want to extend the tube magazine, or convert the gun to use a
Extended tube--possible. They do it on shotguns all the time. It would
probably extend to a foot in front of the muzzle.
Box magazine conversion--also possible. Machine out the reciever,
fabricate a magazine catch and probably some feed rails, build from
scratch or convert an existing magazine.
Bring your checkbook, neither option is going to be cheap.