Yes, you could do that.
you would also have to swap out the local checkout code to use sage pay.
There is a tutorial for that in the how to section of the eCart support page.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.