That model used to be offered with barrels that were fitted with choke tubes--under Winchester's trade name of Winchoke. Maybe there are Winchoke barrels out there for sale--and you should be able to change barrels on that gun without fitting. The other way might be to have a gunsmith thread the barrel for choke tubes, and get the chokes you want from a vendor like Colonial, who makes choke tubes for just about any shotgun ever made.
