Sorry, I don't know the DNN installation...
This property can be add in the CuteEditor tag like this:
<CE:Editor id="Editor1" EditorWysiwygModeCss="SeaEditor.css" runat="server" ConvertHTMLTagstoLowercase="false"/>
or used in the code like this (in C#):
Editor1.ConvertHTMLTagstoLowercase = false;
if that does not help you, it is necessary to await an answer of the team of development... sorry ;-)