I don't think it would be practically possible for the priest to "unlock" sharpshooters as you say. It might be theoretically possible if you attach the advisor before buying docrines (which seems unlikely in my opinion) but I can't see how it could help you.
For sharpshooters to work models, not units, need to be Guard Infantry (GI) with BS3. The priest doesn't make units into GI if they weren't already, so doesn't let you buy sharpshooters for ogryns and so on, and he doesn't change their BS.
As with the Hardened Vets it may be possible to buy the skill for units with BS 4 (or 2 or whatever) but it wouldn't allow BS 4 troops to get the reroll. Why would you bother?