Yes yo ucan, but not all at the same time. When creating a new blank template in Design Studio, you can switch to HTML view. Just copy/paste the HTML code of your previous email. I have done this for purchased/downloaded email templates and it works perfectly.
Just be sure to go back and add in your Marketo editable regions (assuming you want both protected/editable areas of the template). And to optimize deliverablility, your images should be hosted on your domain - if that means within Marketo, you'll need to change all of the image references to the absolute URLs to the images that reside in Design Studio.