debootstrap is a tool which will install a Debian base system into
a subdirectory of another, already installed system. It doesn't require an
installation CD, just access to a Debian repository.

In FreeBSD, you can use debootstrap to install Debian into
a subdirectory of your existing FreeBSD installation (i386 or amd64)
and then run Debian GNU/kFreeBSD (i386 or amd64) or Debian Linux (i386)
in a jail or chroot.

WWW: http://wiki.debian.org/Debootstrap