Hi,
If you do only repacking and no machinery involved in this then create a new separate work center and assign a new standard value key having only labor hours as activity type.
Now in the rework order delete the original operation ( only one in your case) and enter this new work center and give operation description as rework and maintain planned hours for this rework activity..
Now during confirmation enter the actual hours...this will capture your labor hours, and subsequently labor costs..
Now it is better to create a new batch for this rework stock for differentiation from original batch stock.
Hope this helps...
Thanks
Kumar