Before being handled by the distribution-specific packaging method, the templates are merged based on the inheritance rules defined by the config files.
It is possible in a level, instead to completely override the parent version of a file, to just complement it by appending lines to it. This is done by providing a file whose name is suffixed by the $append string. The ordering in the target file of the file parts specified this way is as expected: an append snippet will always follow the initial contents of the file, and will always precede append snippets defined lower in the hierarchy.
Note: I understand this is not really a generic mechanism for combining parts of files, but this was the most needed combination. When other needs are identified, we'll see whether generalizing or replacing this mechanism will be adequate.