You can’t legally increase the magazine capacity of an imported shotgun to more than 5 rounds; doing so violates 922(r).
In order for the shotgun to remain legal, you need to replace 3 imported parts with US-made parts, and finding US-made 1301 parts can be difficult (and why I bought the factory 922(r) compliant model).
And since a mag extension is not one of the ATF-designated “base” parts, it doesn’t count, even if it is US-made.
Your best bet to contact someone like the guys at Freedom Fighter Tactical; they make a lot of Beretta 1301,parts and can tell you which ones count for compliance.
They specialize in the Bempnelli M4 but the concept is the same.
https://freedomfightertactical.com/pages/922r-faq