Upload of ready theme is not supported. But you may create new template, directly from your admin panel. To do that, just click Theme Editor
Next, add all necessary fields, and add theme image. Save it.
Please note, this template do not have any files nor css. You will need to add it.
How to build own theme may find at nopCommerce documentation: http://docs.nopcommerce.com/display/en/Designer%27s+Guide