Sometimes you'll have a third party service, or in-house designer create an email container (graphical headers and footers, like stationery), or an email template (the text and body content), for you to use in your ISN. It is not uncommon to receive this information in something called an HTML file (denoted as filename.html) which looks something like this when viewed:
Here's how to make that into a template in your ISN:
Go to Email -- My Email Templates -- Add New Email Template
Then add a name, subject line, and container (if you aren't using the default), to the top section.
Below the Attachements sections click the < > button.
This will pop up a blank window where you can paste your HTML code into:
Hit Ok when you're done pasting it and it'll show up in your regular template editor:
Scroll down and hit Save Template and you're all set!