Plenty of people have had the idea, but unfortunately there isn't an easy way to convert it without using the ability to use shot due to the spreading effects a rifled barrel has on the shot density.
So unless you want to stick with a dedicated slug Saiga, the best bet for rifled functionality is to use a "paradox" rifled choke or barrel extension. If you can find one, that is! They are usually as expensive as a new barrel
I'm currently working up slug loads and hope to have suitable accuracy out to about 100 yards. If I could find a suitable rifled barrel I would be tempted to sacrifice it to make my own rifled choke extension.