Download the Knak email to your hard drive by selecting Publish > Download HTML from within the email builder.
Open the email, right-click, and 'View Page Source'.
Select ALL of the code. Copy it.
Login to Marketo and navigate to the Design Studio.
Create new email template.
Important Note: Make sure you're in the Email Template section here, and NOT the Landing Page template section. It's easy to get them mixed up.
Paste all the code from your Knak template into the HTML Source of the new template.