Hi Nick,
Unfortunately out of the box there is no easy mechanism to do this.
If this is something you will be doing quite a bit, code a simple upload and 'add to marketing list' aspx page. This page would require custom code, but these are items relatively straight foward to do with the crm sdk. For example - single page where you select the marketing list, then select the file, then click upload.
Alex Fagundes -
www.PowerObjects.com