Thru-hulls that are secured to the hull via a flanged nut and backing board (beneath the nut) can have a ball valve added to the threaded end of the thru-hull fitting. You will need to hold the thru-hull so it will not rotate from below, put plenty of PTFE thread compound on the threads of the thru-hull fitting and tighten it down as many turns as you can get before it binds. Use the biggest pipe wrench you can find/fit into the workspace. The thru-hull does not have tapered threads but the taper in the ball valve is enough to get a good seal.
All of the plastic ball valves that Capital Yachts used below the waterline should be replaced. Ideally all of them should be replaced with bronze or marelon; I prefer bronze. The thru-hulls themselves were heavy duty on my boat (a '79 N-41). The old fittings are twice the thickness of new ones and did not require replacement.