[exim] help with rewrite rules

Hello
I've exim4 running on a debian box, I have a set of processes running under
different users rights, they send email periodically using their user
address, I need to rewrite the sender field of their messages after they
generated them and before exim deliver these messages. To the final
destination, all messages should look like a unique user sended all of them

Thanks in advance for any help
Enediel
Linux user 300141
Debian GNU/Linux