A windows server is connected to a modem and has a public ip address. 2 others public ip addresses are reserved on the server (but not assigned to physical interfaces)such that these addresses are translated into private addresses on other servers on the lan.
Is there a feature on ASA5520 that could make a nat translation on public ip addresses not assigned to the asa physical interfaces but dynamicaly kept in the config file and be able to do nat?
Yes, absolutely an ASA can do this.
static (inside,outside) 18.104.22.168 192.168.5.1 netmask 2522.214.171.124
would present a server with a real address of 192.168.5.1 on the inside to the outside with an address of 126.96.36.199.
The 188.8.131.52 address does not need to be assigned to any of the ASA physical interfaces.