In Lebot's book, Kava: The Pacific Elixer, he refers to earlier authors who suggested that the reason there is no kavain in kava leaves is "the immediate reduction of one double bond (7,8) by ascorbic acid." (p. 67). Reducing that double bond (i.e. changing it into a single bond) would change the kavain into DHK. Lebot was referring to the biosynthetic processes that naturally make kavalactones in plants, but previously on this forum I have speculated based on that passage that mixing your kava drink with vitamin C might cause the same thing to happen. But as far as I know that has not been proven or disproven by experiment. It might be possible, but it's an unproven hypothesis that that reaction actually happens in the conditions in a kava drink. And it's unclear if the role of ascorbic acid in DHK biosynthesis is really any more than a hypothesis either. So I don't have any idea how much vitamin C you would need to make it happen (if it actually does).