It will be, however, the second child for Joel, 65, who shares daughter Alexa Ray Joel, 29, with ex-wife Christie Brinkley, reported Us magazine.
The representative said Joel and Roderick, 33, are expecting their new addition "this summer."
The two started dating in 2009, following Joel's split from his third wife Katie Lee. The performer was also previously married to first wife Elizabeth Weber Small in the 1970s and early '80s.
Joel and Roderick chose not to confirm whether they know the gender of their own child, as the representative said they are opting "to keep any further details of her pregnancy under wraps until their new addition makes an official debut.