You can add your own html to the Source Code in the editable section of a marketing piece. To do this, head to the 'add general content' step of your marketing piece, and place your cursor in the editable area. When the tool bar appears, click Tools >> Source Code. A box will appear where you can edit the HTML of the marketing piece. Feel free to edit, or replace it with your own code. Click Update.
Note: Not all code is created equal. CSS will become different, and the content should be reviewed. The styles could be lost, and those should be reviewed and edited.