Q: Can the base station be an Airport Extreme?

Louly wrote:

 

IMy question is can I extend a network created by an Airport Epress with another Airport Express. All attempts failed as Airport Utility cannot see both in serie.

That's exactly my configuration.  The setup was a snap, starting with a factory default reset on both units.  After the reset, I first powered the main Express and connected it to the modem.  It took a few minutes to turn green and then I accepted the recommended settings in Airport Utility.   I then powered the extender Express and, after waiting for the green, again accepted the recommended settings.

The only thing that I had to do afterwards was to insert administrative and WiFi passwords.

Yes but the text in the body of your post was correct.  In any event, I have two Express units.