One of the most frustrating things about creating a project template is the inability to take an existing project and simply save the project as a project template. Go ahead and check it out... if you choose File > Save As, you won't see template in the Save as type drop-down menu.
There is still hope, however, if you'd like to use an existing project as a project template... and it's a simple workaround. Find the project file on your computer (it will have a cptx file extension which is typical for a Captivate 5 project). Simply change the cptx extension to cptl, which is the extension used when you create a new project template from within Captivate.
When the time comes to use your new template, return to Captivate and choose File > New Project > Project from Template. Navigate to the location of the file name you changed and you'll be able to create a new project based on your new template. You'll notice that the resulting project will be untitled and contain a cptx extension, just like you would see if you had created a project using a template created from scratch.
Note: The one downside that I've seen using this workaround is that you won't be able to add placeholder objects or placeholder slides to the cptl file that you created by renaming the file outside of Captivate. Other than that, the template seems to behave just fine.