The data folder is the preferable place to add new files to ToME or T-Engine. Note that there is a special way to refer to the path when referring to files in your data directory. For instance, if your addon is named "foo", to load a talent definition from its data directory you might refer to "data-foo/talents/...". There is no folder named data-foo: the actual directory structure would look like "data/data/talents/..."
Note that some files which seem that they should be data, instead generally need to be overloaded. Many graphics files, for instance, are referred to with a filename, rather than a full path, and T-Engine code then prepends the proper path. Unless you want to modify those instances of the T-Engine, these kind of files will typically need to be overloaded instead.