There is the /etc/sudoers.d directory in recent Debian versions:
pabs@chianamo ~ $ sudo cat /etc/sudoers.d/README
#
# As of Debian version 1.7.2p1-1, the default /etc/sudoers file created
on
# installation of the package now includes the directive:
#
# #includedir /etc/sudoers.d
#
# This will cause sudo to read and parse any files in the /etc/sudoers.d
# directory that do not end in '~' or contain a '.' character.
#
# Note that there must be at least one file in the sudoers.d directory
(this
# one will do), and all files in this directory should be mode 0440.
#
# Note also, that because sudoers contents can vary widely, no attempt is
# made to add this directive to existing sudoers files on upgrade. Feel
free
# to add the above directive to the end of your /etc/sudoers file to
enable
# this functionality for existing installations if you wish!
#
# Finally, please note that using the visudo command is the recommended
way
# to update sudoers content, since it protects against many failure
modes.
# See the man page for visudo for more information.
#
pabs@chianamo ~ $ sudo tail -n3 /etc/sudoers
# See sudoers(5) for more information on "#include" directives:
#includedir /etc/sudoers.d