It can be reapplied in ZBrush. First, make sure that it’s still 16 bits in Photoshop (only Photoshop CS supports that color depth correctly), then export it as a PSD.
In ZBrush, load your head model and import the displacement map into the Alpha palette. Apply a texture to the model (even a blank one), turn off Quick 3D Edit, turn on Tool>Display Properties>DSmooth with a value of 1, and activate Tool>Displacement>Mode. Go to the highest subdivision level of the model (make sure that there are enough polys to support the details that will be applied via the displacement map), and set the Tool>Displacement>Intensity slider to a value that looks right on the model. When ready, press Tool>Displacement>ApplyDispMap and presto! The displacements will be converted to actual geometry, ready for you to continue sculpting the model. When done, you can create a new displacement map via the standard technique.
Enjoy!