You can specify a file to import using "filename.zip!data.xml" where it will then read the file data.xml from the filename.zip.
In an imported sub-template, if its properties for a style (ex. Normal) differ from the parent template's properties for the style, the use in the sub-template can be set to either use the properties in the sub-template, ot the properties in the parent.
All file operations, including reading an imported file, use a Windward access provider. Access providers exist for all the common protocols (file servers, basic, digest, OAuth2, etc.). And if you need something different, you can write your own access provider - once. And it then handles any access of files using that protocol. (ref: Ohana)
An import tag in your template imports a sub-template (or bitmap). The imported template is processed as though it was inserted in-line at that point. All tags in the imported template are processed. All variables in scope at the import point are available to the sub-template.