Can I include content from an external website into my email?

Generally not best practice as there will be rendering issues. Options are in this order from best to worst: develop it into the email, link from email, include content as a picture.

We have an idea in the community for this - please vote it up

http://community.marketo.com/MarketoIdeaDetail?id=08750000000HAdHAAW

I can't think of a clean and scalable way to achieve this programatically.  If you don't update content very often, it is probably best to duplicate the effort on both the website and email.  Below is what I can think of but someone else on the community might have a better solution.

Set up a My Token of type Rich Text for your Email Blast Program and apply that token in your email.  Next write some code to access your XML based content and apply whatever stylesheet transformation to make it look pretty in the email.  Next call Marketo SOAP API to set the Token.  So everytime you change the content, you can run your code/script to update the My Token in Marketo and your email should be updated with the latest content.

As you can see, it takes quite a bit of effort to achieve this.