Both will play regular roles if the NBC project goes to series, according to Deadline.
Wittels, who previously collaborated with Silverman on her Comedy Central series, will play a fictionalised version of himself in the pilot.
The actor previously played a recurring role on Parks and Recreation and has also written six episodes of the NBC comedy.
Party Down actress Raphael will play an old friend of Susan, the character played by Silverman, in the pilot.
She currently stars on Adult Swim's crime drama spoof NTSF:SD:SUV and wrote the screenplay for 2009 film comedy Bride Wars.
Silverman's pilot, which will see her newly-single character struggling after a decade spent in a relationship, landed at NBC in August after reportedly sparking a bidding war between the television networks.
Other comedy projects currently in the works at NBC include a show from Glee's Ryan Murphy, a puppet comedy from the Jim Henson Company and a new sitcom from former Seinfeld writer Spike Feresten.
> Marcia Gay Harden to star in NBC's Isabel