You can insert any HTML element into HTML Editor's content on client side.
See InsertHTML function description at Main functions and interfaces page.

If you don't want this inserted element's properties be edited with Context Menu, you should set special attribute in this element.
This attribute is named obout-no-contextmenu and it should be set to "on" value.

Sample

<script type="text/JavaScript">
function putToEditor()
{
 oboutGetEditor('editor').InsertHTML
               ("<div obout-no-contextmenu='on' style='width:300px; height:100px;'>some text</div>");
}
</script>

After this function inserted DIV element into HTML Editor's content, you can not change its width and height because Context Menu will not offer you "DIV properties" item (in design mode).