Removing mirror declaration from /etc/apt/sources.list in favor of a proxy in /etc/apt/apt.conf.d/02proxy

Is there a way to tell Foreman via the ubuntu preseed add append the
apt-mirror proxy to /etc/apt/apt.conf.d/02proxy instead of all of the
lists inside /etc/apt/sources.list?

The reason I am asking is because the proxy is a better solution. It will
work for PPA packages and any other arbitrary thing added into
/etc/apt/sources.list.d/. However, the two options are incompatible. I
can't seem to declare both a mirror in /etc/apt/sources.list and a proxy in
/etc/apt/apt.conf.d/02proxy. Or maybe my implementation is wrong?

ยทยทยท -- Stephen Wood