For Kali we are mirroring Debian albeit only on a few architectures:

conf/distributions:

    Codename: debian-testing
    Architectures: source i386 amd64 armel armhf
    Components: main contrib non-free
    UdebComponents: main
    Update: - testing
    SignWith: 7D8D0BF6
    Log: debian-testing-changes.log
    Origin: Debian
    Description: Kali's mirror of Debian's testing release

conf/updates:

    Name: testing
    VerifyRelease: 8B48AD6246925553
    Method: http://ftp2.us.debian.org/debian
    Suite: testing
    # This filter drops the linux-* package that we maintain ourselves
    FilterSrcList: install filter-packages-from-debian
    DownloadListsAs: .xz .lzma .bz2 .gz .

Unfortunately this setup results in lots of cruft because it mirrors source
packages that do not have any associated binary packages.

$ reprepro reportcruft debian-testing
[...]
source-without-binaries debian-testing arcboot 0.3.15
source-without-binaries debian-testing arcboot-installer 1.24
source-without-binaries debian-testing arcload 0.5-7
source-without-binaries debian-testing colo 1.22-1
source-without-binaries debian-testing cuse4bsd 0~svn2434-2
source-without-binaries debian-testing eglibc 2.19-3
source-without-binaries debian-testing freebsd-quota 8.2-3
[...]

It would be nice if you could enhance the update mechanism
to filter out those source packages.

Cheers,
 Raphaël.