Hi and Welcome to Auctiva. All your activa listings will be imported to Auctiva within 12 hours. If you want to change those listings to add a template you'll have to create the add here and then get the html (there is a button) and revise them on ebay. sorry there isn't a easier way to do this.
One word of caution, Auctiva checkout does not do combined invoices, so if you usually sell more then one item to a buyer, I'd suggest turning off auctiva checkout and using ebay checkout. They are suppose to be adding this feature in the future.