I'm not quite sure what you are asking, but let me take a guess (I'm assuming you know OSLC).  The primary text in DNG is stored as HTML.  So you would convert the RTF text to HTML before inserting it into the primary text.  You can use something like the RESTClient plugin in Firefox to test things out first.  I would recommend creating an artifact with empty primary text first so that you can see what it looks like (starts and ends with <div></div>) and then insert your HTML text in between the start and end markups.  You may have to modify the RTF to HTML conversion text based on what is produced by the conversion tool and what is allowed in DNG.