You cannot copy between dissimilar model routers.
And there is absolutely no reason and no need to turn off wifi in the current router and every good reason to leave it on.
The extreme should be in bridge mode which Apple setup to remove the guest wireless. So I would encourage you to simply run two wireless networks.. (there is bandwidth to run 3 without causing issues).
Create the wireless on the Extreme with your own new wireless values.. and connect whatever Apple devices you want to it.
But you can do what you want.. simply use the existing router SSID in the Airport and use its password.. however this can cause issues in the clients that expect one setup and find they are connected to something unfamiliar.