You can manually add the templates that you wish to use when sending an email campaign. To do that:
Click Template List from the Navigation Toolbar to the left.
Click Add Template in the top-right corner.
In the slide-in window that appears, specify a Name for the template. The template name can only contain letters, numbers, underscores, hyphens, and spaces. No other special characters are allowed, such as ! @ #.
Choose between Text or HTML for the Template Content Type.
Based on the type you choose, paste the content under the Template Content text area.
Click Add to create the template. 
Once a template is created, a unique Template ID will be generated. You can copy this ID to use it when sending email campaigns via API.
You will not be able to preview the template at this time.
Other Actions
At the bottom of the Template List module, you can change the number of templates that can be displayed on a single page. Click on the drop-down field to change the count.
At the top-right corner, you can search for templates by typing their name in the search bar. Once you've typed the keyword, click Search or press enter to display the results.