[Exim] file lookups with dynamic content....

Hi,

I'm trying to set up a mail2news gateway.
In this transport I pipe the mail to the gateway script via the
command option.
Like "command = /opt/mail2news/mail2news rzv.lists.exim-users"
whereas the first parameter (rzv.lists.exim-users) is the newsgroup
the script has to insert the message into.
Now my question is:
Is it possible to build up a file with dynamic content based on the
message headers like this:

$sender = exim:    rzv.lists.exim-users

??
I only work with file lookups where the contents of the files are
static like

exim:     rzv.users.exim-users

So the idea is to have at the very end a command statement like

"command = /opt/mail2news/mail2news lsearch;/etc/exim/mailingslists"

so that I can handle several lists with just one transport, given
that the dependent router listens via a condition for one special
email address.

Thanx a lot for any hints or links.
If I didn't point out my problem good enough please let me know so!
;-)

Michael