Build-Depends on source of a different package?

[Andreas Metzler <ametzler@logic.univie.ac.at>]

Hello,
Is there a right way for using the source of a different
debian-package?

I've checked the archives, but I only found 
http://lists.debian.org/debian-mentors/2001/debian-mentors-200103/msg00253.html
which is a little bit dated and without really positive outcome. The
best suggestion was something similar to
if [ ! -e $TMPDIR/foo.tar.gz ]; then
   pushd $TMPDIR
   apt-get source --tar-only foo
   popd
fi

If you're wondering: The package in question would be exim v4 with the
exiscan patch applied. - It cannot simply be built from the exim4
source-package because the patch is released more often than exim
itself (current-version is already 4.12-24).
               cu andreas