Job done
Since there is no official way to do this here is how I did it. Maybe Apple should take a look at this, check it and publish it somewhere.
1* Enable Internet sharing from Ethernet 1 (PPP) to Ethernet 2 (Airport Express).
2* Manually configure IPv4 on Port 2 with address 192.168.2.1. Submask at 255.255.255.0
3* In Airport Utility create a wireless network. Setup connect via Ethernet.
4* Share connection should be deactivate (bridge mode).
5* Manually configure IPv4 in menu TCP/IP.
IP:192.168.2.12
Subnet mask:255.255.255.0
Router:192.168.2.1
DNS:192.168.2.1