To make a template from an HTML file, select Email Campaigns > Create/Edit a Template or navigate to the Email Campaign tile on your Home Dashboard and choose Design or View a Template.
Select Use a Blank Template from the template folders.
Highlight the default text and delete it.
Now choose Source and highlight and delete the text.
Paste in your HTML Source Code. Press Source again.
To complete, select Save and exit and name your template. Your template will now be located within the Private folder.
Read: Creating a Template using HTML