You can update the associated .js file in html_editor/custom/ and change the enter mode from:
FCKConfig.EnterMode = 'p;
FCKConfig.EnterMode = 'br';
FCKConfig.ShiftEnterMode = 'p';
The editor will automatically add tags when the enter mode is p or div. You could also try changing it to "div" and it will add a div tag instead of a p tag and that might work better for you.