If you want to extend the range of your AirPort Wifi network, you can add more base stations. In the configuration dialogs, the option to “Extend an existing wireless network” is advertised, but this is intended for wirelessly connected base stations. This option affects your network throughput. If you have a wired network between your base stations, you can create a so called “roaming network”, although this is not advertised.

The gist is the following:

1. Select “Create a wireless network”
2. Enter the same network name, security and password

Apple has a support article with more details.