Any package preparation tools can append to a substvars file; just make sure, if you're using debhelper, that you do it after dh_prep
gets called, since that will erase any debian/*.substvars
files lying around.
You can even use your own substvars file if you like, if you're worried about interfering with debhelper (or debhelper interfering with you). Just add the substvar filename to the arguments for dh_gencontrol
with something like this:
override_dh_gencontrol:
dh_gencontrol -- -Tpath/to/my/substvars
See man deb-substvars
, man dpkg-substvars
, and man dh_substvars
for more information.