connect airport extreme to ethernet

You posted in the iPad in the Enterprise forum. I've asked the Hosts to move your post to the AirPort forum where you'll be more likely to get assistance.

Regards.

Can I connect an Airport extreme to another wireless router?

Yes, if you use an Ethernet cable to connect from a LAN <-> port on the existing router to the WAN "O" port on the AirPort Extreme, and then configure the AirPort Extreme according to your needs.