You have several options here:
a) You can fork/modify the upstream module, but that is probably not a very good idea if you want to update that module in the future
b) You can create a dummy module that contains only an empty init.pp and your conf file. This is somewhat ugly from a puppet perspective but should work.
c) You can switch from the puppet:// protocoll to file:// or http:// and store the file on a universally acessicible network filesystem or webserver.
Those are all the options I can currently come up with, maybe there are more.
Still, this is more a Puppet related question than Foreman. Foreman just helps you to get the variable set.