Each set of 30mm rings comes with two sets of 10moa inserts. So you can get 20moa out of the two sets by putting the -10 insert in the bottom of the front ring and the +10 insert in the bottom of the rear ring.
They do have any other offset inserts for the 30mm.
If you need more MOA you can use a tapered base with the signature rings. I used the Leupold LR STD base with Burris 30mm STD Signature rings on my Remington 700P to create (in theory) 40 MOA. I figured it out and with the comes up I had I could crank out to 1525yds, with a 300yd zero.
I guess you could also use the Signature Zee rings with 20MOA Picatinney rails. JohnnyK.