Rewrite the descriptivepart of a sender address ?

Hy,
i manage locally generated email through sender_canonical_maps:

#cat /etc/postfix/main.cf ! grep sender_canonical_maps
sender_canonical_maps = hash:/etc/postfix/sender_canonical_maps

Here is sender_canonical_maps:

#cat /etc/postfix/sender_canonical_maps
root@the-local-hostname.priv <address@realdomain.tld> (Root of
the-local-hostname.priv)

#postmap sender_canonical_maps
#/etc/init.d/postfix restart

Sending email as root results in receiving email as:

From: address@realdomain.tld (root)

Instead I want:

From: address@realdomain.tld (Root of the-local-hostname.priv)

Is there a way to rewrite not only the real address, but the descriptive
part too ?

Thank you !

--
Diesis