[Exim] applying procmail to every incoming mails?

Hello, I am using debian woody box running
exim but my old procmail filtering for every incoming mails
does not work because the exim.conf is not properly configured I think.

I have the following lines in my exim.conf
---
# This transport is used for procmail

procmail_pipe:
driver = pipe
command = "/usr/bin/procmail"
return_path_add
delivery_date_add
envelope_to_add
# check_string = "From "
# escape_string = ">From "
suffix = ""
---

The .procmailrc in the user's home directory works very well
but I would like to apply the systemwide /etc/procmailrc to *every*
incoming mails to filter spams but don't know how to configure it.

Can anyone please let me know how to combine procmail to exim?

Thanks very much.
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