Many of these older lifeboats had some inside ballast, but because these boats were never intended for inside ballast, it will greatly shorten the life of the boat. But more to the point I think you are really asking how to convert these old lifeboats from what was essentially a rowing boat to a sailing vessel.
You have to start with the basics. The framing on a wooden sail boat is somewhat different than the framing on a rowing boat. While both will have transverse frame and floor timbers (a term which refers to the heavier frames that cross the keel and tie the two sides of hull together). In the case of a rowing hull, the floor timbers only have to keep the two sides of the boat together. But in the case of a sailing vessel, the mast, keel, rudder and rigging loads are trying to twist the boat apart and the floor timbers have to absorb that twisting force and distribute into a large enough area of the boat to prevent the boat from damaging itself. Typically the bolts from the external ballast keel pass through these floor timbers and so the timber has to be thick enough that the bolts won't split the timber apart.
Therefore retrofitting a lifeboat to be a sailing vessel therefore requires adding deeper, heavier floor timbers.
Adding a ballast keel means that there is a load perpendicular (vertically) to the keel as well and a lifeboat keel was not designed for that load so a keelson (inner keel timber) is added to distribute the load fore and aft.
When you see chainplates bolted to the outside of the hull of a wooden boat, they are typically bolted through a heavier frame on the interior of the boat. This frame typically has a hanging knee that ties that frame to the deck and sheer clamp.
Lastly while woodenboats are typically keel stepped, the keel of a lifeboat was not really designed to take the vertical thrust of the mast and so a timber gets added above the keel that distributes the loads two three frames fore and aft of the mast step and of course big floor timbers are added here as well.
Once you have done that, you can easily add a lead shoe, or a lead fin keel.
Once you have done all of that then the lifeboat will have the strength needed to be a sailboat and if it has a proper rig, sail moderately well, but not great.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies