Templates are only used when you are creating a new document. When you open a file, no template is used, everything comes from the document alone.
So, the workaround is to first create a new spreadsheet document using your template, then use Insert > Sheet From File to import your csv data.
In that process, the template can only provide settings and cell styles; the data from the csv will not mix with content and formatting loaded through the template. If your goal is a spreadsheet that mixes content from the template with data from the csv, you'll need to take a different approach.
AOO4/LO5 • Linux • Fedora 23