One way to apply a template to one page is by creating a separate Web site in XSitePro. You would have to publish this page to a different directory on your server.
Another way is to use "Ignore Page Layout Completely." Copy the code from one page of a template (using the Source view) and paste it into the Source view in the page where you are ignoring the page layout.
NOTE: With both solutions navigation is an issue.For method #1 above, you can use External Links in the navigation and use the "Link to another page in XSitePro" option.
Here's an example from one of my sample sites:
http://www.xsiteprotemplates.com/dogdayz. Click on Today's Special. That link takes you to a sales page that was created by using method #2 above. I have simply added a link back to the Home page to deal with the navigation issue. I could have added a manually-coded menu to all the other pages in that site, but they would not get updated automatically by XSitePro. If you wanted something really professional for the menu, you could use a third-party solution like
AllWebMenus Pro (affiliate site), but the pages would still need to be added manually to the menu.