Karl is in one episode in Season 5, as Kor in "Lifeblood". He's listed as appearing as Julius Caesar in "When Fates Collide" in Season 6. I'm not sure how big a part he'd have in that one with Caesar being dead and all.
Karl is also in one more Hercules episode, playing Caesar in "Render Unto Caesar." That's in Season 5, which should also be available in the US by the end of the year.
From what I can tell all the Xena seasons are available in the UK and Australia already. The US fans are the ones having to wait for a change.