It would be best to convert the template to a dreamweaver theme:
You can them apply the theme that you create to the Data Assist pages, and add the eCart add to cart buttons.
The question you ask is fairly broad. I would suggest starting with the eCart getting started guide to become familiar with using eCart.
From there experiment with the Data Assist Create pages wizard of Data Bridge to become familiar with data assist.
once familiar with data assist and eCart start working on adding eCart to data assist pages and finally adding the theme.