
From [5] of Angular KineticsParticle and Figure 1:
where
The total angular momentum of a system of particles has two terms: the angular momentum of the CM (the first term of [1]) and the sum of the relative angular momentums of the particles to the CM (the second term of [1]). Unfortunately, the angular momentum of the system does not reduce to that of the CM alone. This equation will be revisited in the Rigid Body Dynamics section. 
© YoungHoo Kwon, 1998 