We have this debate quite often on a CA based forum with input from lawyers. It sucks that we have to play this game but that is exactly what the CA DOJ does, play word games with the penal codes or ever worse make up fudd and spread it to our local PDs.For P99 mags, the feed lips are (w/o alteration) too wide on a .40 S&W mag to retain 9mm rounds. I vaguely recall someone trying this and being unsuccessful. That said, if this is being asked to find a way around state laws regarding capacity limits I suggest using caution here. The authorities probably will not be amused if you cram 11 or more rounds of 9mm in a 10 round .40 S&W mag. -Be careful.
CA Penal Code 12020 is available online as is the CA DOJ's interpretation of it. The penal code has a broad prohibition against detachable magazines with a capacity that exceeds ten rounds made or imported after January 1, 2000. While there are exceptions (notably for law enforcement and licensed dealers), regular CA residents are prohibited from making them, getting them, and transferring them to other CA residents. Not surprisingly, CA DOJ has the following interpretation of the penal code:The PC specifically states that one can't 'sell, import, or manufacture' a hi-cap magazine. Seems they left out buy but that is up for the DA to decide.