One way would be to make a duplicate file, similar to the /etc/virtual/pophosts file.
Make your own, such as /etc/virtual/relay_ips, then edit the /etc/exim.conf line that says:
hostlist relay_hosts = net-lsearch;/etc/virtual/pophosts : 127.0.0.1
to
hostlist relay_hosts = net-lsearch;/etc/virtual/pophosts : /etc/virtual/relay_ips : 127.0.0.1
and restart exim. You'd put each IP on a line in the relay_ips file.
John