Dear Shengtao,
Yes, I agree that you could augment PtfmRIner, PtfmPIner, and PtfmYIner with the missing inertia terms, if they are important. More information on this limitation is given in my Jul 11, 2014 post in the following forum topic: AddCLin for OC4 Semisubmersible - #3 by Jason.Jonkman.
Best regards,