Wonder what that means? Not sure how you'd modify a magazine to make it hold more rounds. You could buy an extended magazine or you could tape two magazines together butt to butt for a very quick reload but could easily be counterproductive with a pistol.
Weld on some sheet metal, extended spring. Shucks, anybody can do it. Of course buying one at an evil NRA sponsored gun show and AR give away would be easier.
For glocks, you can get a +2 adaptor.
However, they make an extended length adaptor for use in the KRISS weapons system. It doubles the magazine size.
My guess is that when the reporter said he “modified” the magazine that he simply used a normal capacity magazine instead of a restricted one as his legally required in California.