[imp] Newbie question: Hacking From address of IMP

[Bianchi Quota Leonardo]

Hi, I have a running FrontEnd IMP which talk with uw-imap box.

Horde-2.2.3-1 ; IMP Version,IMP: 3.2.1,PHP Version; PHP Version: 4.2.2;
IMAP (imap-4.5-4 on a RH6.2)

I need configuring IMP to manage two domains, the old one which is
already working and the new one.
Now about users, they all appear into the same domain and IMP is good if
reading emails.
The problem rises when users send emails which appear all sent with the
same domain.
 
Then the questions are: 

- it is possible to set From field getting it from a user preference
file?
- where do I set the variable From?
  (this question could help me if I would like to feed from address from
a map file I load from aliases)

Any other suggestion?

Bye,
Leo.