<p>At work we use Sandcastle for creation of help files. I have been using SandCastleGUI for some time and I'm looking for a way to create additional pages in the help file. 
These pages are written in XML format called MAML. 
The only problem is that I couldn't find any decent editor for these file format.</p>

<p>I'm looking for a WYSIWYG editor to create &amp; edit additional documentation pages.</p>

You could use a generic XML editor with WYSIWYG support like
Oxygen
or
Serna
. You would need a Xml Schema or DTD for MAML, I assume there is one somewhere in an SDK or such. Probably the harder part is that you would need a stylesheet that renders the XML to an display format that can be used by the editor to provide a WYSIWYG view of the document.
It works rather well for standard XML formats such as Docbook, but I don't know how easy it is to find/create the needed stylesheets for MAML. But generally there is no reason why it couldn't be done.