Okay, I'm gonna try to list it step by step so I hope you don't mind if some are just too basic. (I'm using Poser6 SR2 btw)

Import the original hair geometry as obj with everything unchecked.

Morph the obj with magnets, but do not move the object.  If you are fitting this to V4, I would move the V4 figure instead.  Don't parent the object either.

Once you morphed the obj, and I'm assuming the morph is only done to the head portion of the hair, click on group editor and select the group head.  Make sure the group is highlighted and it is the correct group that you made changes to.  Click Create Prop and name it as V4Fit, or whatever.  Inspect your new object with the morphed object to make sure the changes have indeed carried over.  Export out this new obj with only the last option checked, "As Morph Target", and save it as V4Fit or whatever.

Load the hair figure into the scene.  Select the head portion of the figure.  Under Properties tab, click Load Morph Target, and load V4Fit as morph target.

Above step was only for the head group.  If you've made any changes to the tails or other parts, follow the same procedure as above for each group.  For example, if you made changes to lTopBraid portion of the hair obj, then you would select the corresponding group under group editor, and create a prop using that group.

Since this involves only head group, the process is pretty quick.  But other objects with multiple groups it becomes really time consuming, if you've made changes to more than one group.  I could swore someone made a python script for this but can't think of the link at this moment.  Maybe someone can chime in on this.

Anyway, I hope this works out for you, and let me know if there's a problem.