>Is there a way to add a new viewer under tools/folder options/files types and configure DDE for it?

You can add handlers to a given file extension by opening up HKEY_CLASSES_ROOT\. file extension; if it exists, it'll point to the name of a handler for the file extension. Under HKEY_CLASSES_ROOT\handler there will be a number of shell entries that specify a verb, the executable to trigger and optionally a DDE directive if the server supports DDE (not all do, or the default may not offer the one you want.)

Your best bet is to follow an existing extension through the registry to see how the keys are built.