You want to do the same thing as if you are recuring your rock.put all your rock, including your new peice in a big container, 80deg, no light, lots of flow, let run for a few days, check ammonia level, they will be high, do a water change, and repeat the process till there are NO ammonia levels. it is going to take a little bit of time to do it right, Dont rush this.
this is how i did it when i made rock and it worked out great..
did I miss anything? anyone?