Replacing or adding keel bolts is a pretty major undertaking. Because keel bolts are bent in a J before the final lead is poured, they are actually cast into the top layer of lead. This makes drilling holes for new ones a little problematic, since if you hit one the hole you just drilled is going to have to be filled and relocated.
Mars Keels has a service where they replace keel bolts, but it takes removing the keel and shipping it to them. I have never done it, but I would guess it is a pretty expensive undertaking. But they do have instructions online for how to do it. http://www.marskeel.com/fckeditor/us...cement_pdf.pdf
The first step in this process is the same regardless if you do it yourself or pay someone else too. You have to take the boat out of the water, then remove the mast from the boat. Once that's done you unbolt the keel, and have the hull moved to new stands in the yard. This allows you to inspect the keel bolts and determine which ones are candidates for replacement.
It is a scary process the first time through it, but it isn't really that bad. I got a lot of practice working for a racing boat that switched keels regularly. About every 3 months we would swap out the keel for a different one.
Personally if I did go this route, I would switch to titanium bolts (not my sig, this is a conflict of interest for me). Once replaced with titanium, things like crevice corrosion cease to be a concern for the life of the boat. They are a hit more expensive, but since it is a permanent fix, you will never have to do it again.