I think that you are going to have to add the doppler before the hrtf.  Once
you have hrtf'ed something, it is in a sense in a different file format, since
it has already been correlated to a head.  Thus adding something later is like
adding a sound after it reaches your ears.
	As for doppler equations, you can just solve for the relative velocities of
the the emitter and listener.  Then use the speed of sound in whatever medium
you are listening to convert the velocity to a ratio.  This is the amount of
pitch shift you hear.

	speed of sound + velocity
   ----------------------------		=	ratio of pitch shift
		 speed of sound

	You hear the pitch shift change as the relative velocity changes (like when
the object passes you and it shifts from a positive to negative velocity).
-- 
Mike Berry
mikeb@nmol.com
http://www.nmol.com/users/mikeb