If there is enough metal at the barrel to allow, the barrel is reamed and threaded to allow the use of of screw in chokes. This job is best done by a machine, but some smiths will do by hand. Either way, once the job is done, you should shoot the rifle for point of impact to confirm that the choke system is straight with the barrel, and not canted causing the rifle to shoot off bead.
If the barrel walls are too thin for even a thin walled tubes, the fixed choke can be changed. Full chokes can be opened up(reamed), open chokes can be changed by back boring the barrel before the choke to tighten a pattern.
Also, there are chokes that can be added to the end of the barrel that allow you to tighen/open up your pattern by simple turning the choke to open/restrict the sleave insert. The "Cutts" choke is one of them that have been used for a long time.
Depending on what shotgun you have, you might be able to just buy a new/used barrel that already has the factory choke system for the same price of having the system installed on the old barrel.