[exim] Relay with copies?

Hello,

we are operatiing an authenticated relay and one user is asking us to
send him copies to a fixed email address for email he is sending. How
can I configure this? I would have to add a recipient at the dnslookup
router, which looks pretty like the default:

dnslookup:
driver = dnslookup
domains = ! +local_domains
transport = remote_smtp
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
no_more

But how could I add a recipient there?

Regards
Marten