I would check around to see if you could find another set of barrels with the chokes you'd like. They're out there, but I will warn you that they a) won't be cheap, and b) will likely require gunsmith fitting to get them "on face" to your gun. 

The Superposed is becoming a rather valuable shotgun, depending on the grade of the shotgun. The Superposed was made in Belgium by FN for Browning, and these are considered collector-value guns in the higher grades and better examples. Sadly, the parts available for Superposed repair are drying up, and if it is damaged, you might be waiting a long time to locate some parts.  You wouldn't destroy its value completely by having screw-in chokes fitted, but it would take a heck of a hit. I cannot tell you how much without seeing the gun, the grade, and the condition.

If you've got a 12 ga, you could put Briley fitted 20 ga tubes into the barrels and hunt with it that way.