I've recycled code from the ODT import plugin to create a simple epub template plugin and attached a beta version.
What the plugin does is add all files in its resources subfolder to the current epub. (It won't overwrite existing files with the same name, though.)
Simply install the plugin, display its resources folder in the file manager and copy all files that you want to add to the resources folder. (You can't use subfolders.)
To display the resources folder:
- Press Windows+R (Run)
- Copy and paste %USERPROFILE%\AppData\Local\sigil-ebook\sigil\plugins\EpubTemplate\resources and press Enter. (I've already added a dummy cover page for demonstration purposes.)
To run the plugin select
Plugins > Edit > EpubTemplate.