I did this, but now have two extreme BS. You do what you have done with the AEBS remaining your router. From your other ethernet port run an ethernet cable to your ABS, I had it out from my AEBS about 75feet. You can, of course, have an intermediate switch/hub if you have hardwire computers on your network, is does not matter. On the ABS you will uncheck the "distribute IP Address", under the network tab. Name the ABS network the exact name you have in your AEBS network name, and I believe that is it.
Good luck