you don't need any plugin to use this. Go to Configuring an Oracle SALT Application and scroll down to "Defining the SOAP Header". I think you probably have read it already because your example is directly copied from there.
However the way this works is you simply put the XML fragment in the TA_WS_SOAP_HEADER (for outbound) or read that field from the FML32 buffer to get that fragment (for inbound: note that this limits this ability to FML32 typed buffers as is mentioned in the link above).
Let us know if you need more details.