The length of the barrel really has nothing to do with it since the OP is speaking of an 18.5" barrel. Modern powders burn up an impart there full energy to the shot column before that point usually with in 15". " Yes" you could have a modified choke added to this shotgun if as Guntech says there is sufficient barrel thickness there for the installation of a threaded choke tubes. The barrel can also have a fixed choke sleeve installed. Or like Why Not suggested you can have a Poly Choke installed. Poly Chokes are old school technology since the advent and perfection of screw in choke tubes. I had a poly chokes installed on a barrel 40 yrs. ago. The chap that installed it never got it concentric with the barrel, in other wards she was on there a bit crooked which was not noticeable to the eye. But at 40 yds she would pattern 8" right and 20" low. Rather than play the barrel bending game I cut off the Poly Choke shortened the barrel to 18" installed a set Remington Rifle sights and it became a slug barrel. Then I bought a new 28" mod barrel for the gun. So be careful with poly chokes and who installs it. If I were you I would stay with a screw in tube or the sleeve. Then you have to decide if your going to shoot steel or lead as the taper of the tube will be different for each. Then there is the cost, a Win, 69A is not a expensive gun, it will cost the value of the gun to have this work done.
Anyways Steve Milton who owns Precision Arms in King City Ontario is the most knowledgeable fella I know on the subject of shot guns and choke tubes. He makes his own brand of tubes. Google Precision Arms for all the info. He has done work for me on many occasions over the years, he is very good at what he does. He is also a character and calls a spade a spade in no uncertain terms, so be prepared for that.