user masquerade in postfix

hello, does anybody know what i must do or insert in main.cf to masquerade a user in the address of user@firma.de? i have read many about domain masquarde, but i'm not able to find something about user masquarde - by the way - i know how this can be made in qmail but i want do that in postfix. is it support by postfix or must i play with perl? A. Groll