You can use FL sizing dies to neck size very easily, no specialty dies
needed unless you load for both an automatic and a bolt gun in the same
caliber and want separate dies adjusted for each.
To neck size with a FL sizing die:
Put the case in the shell holder, raise the ram and screw the die down until it just makes contact with the case mouth
Lower the ram and screw the die in 1/2 turn and run the case into the die again, you will see a "ring" where
the die has made contact with and sized part of the case neck and left the rest unsized.
Continue to slowly screw the die down in small increments until the ring of unsized case neck is pushed down until it just before it makes contact with the shoulder.
At this point your die is not touching the case shoulder, and has sized only the neck of the case, lock the die ring in place and you're ready to go!
Note: you may have to adjust the decapping/sizing stem down a little to make up for the die being higher up in the press than set for by the factory, you'll know if this needs to be done if primers don't pop completely out of the case when neck sizing.
TL;DR Neck sizing has been done long before specialty dies were made. A special neck sizing die will quite easily and irreversibly ruin your brass if you adjust it wrong, instead of just full length sizing a case and putting you back at step one (fireforming the case to the chamber).