You could do a general surgery residecy first and then do a neurosurgery residency I suppose...but that would be about 11 years in residency. There is no neurosurgery "fellowship" after general surgery; it has it's own residency of 6 or 7 years of which the first year is general surgery.
You can apply to complete a spine fellowship, post residency. This is done excllusively by neurosurgeons with a couple of orthos mixed in who are going into their attending lives as spine specialists. Otherwise, its like mpp noted, a gen surg intern year followed by the nsurg residency.