We are working on a brand-new template system that will allow you to create your own HTML newsletters. You will be able to edit the HTML in your own preferred HTML editor, such as Dreamweaver. Or you will have the option to edit the HTML directly inside word press.
There is a workaround for using your own HTML and send press, but with the soon-to-be released template system, I don't think it is worth the effort for you at the moment.
Once the template system is released, the only limitations to using HTML are that certain email clients do not handle some standard HTML properly. You'll want to test your own HTML against the various different email clients to make sure they render properly.