If you need a .deb customized on the fly
mkdir /tmp/bb cd /tmp/bb apt-get source busybox sudo apt-get build-dep busybox cd busybox-1.20.0/ fakeroot debian/rules build make -C debian/build/deb/ menuconfig # enable passwd fakeroot debian/rules binary but probably the best would be to add a custom package inside debian/control and the relative config under debian/config/pkg/
(I'm not using Debian 7 but guess it is similar)
edit
You can use fakeroot debian/rules debian/build/deb/.built and fakeroot debian/rules binary-arch_busybox to build the deb target only