we added 250kg to the bulb of one of a 36' race
boat i had a life time ago. the exisiting bulb was a semi-circle, with a flat bottom. the shoe we added complete the circle
transformed the boat and was a major success
the boat already had a seriously overspec-ed aluminium girder sub-floor that the keel was attached to, plus a sister boat had the same mod done a year earlier
this meant we were fairly confident in the engineering, and that we weren't going to rip the keel off
before adding any weight to your keel, you need to do the same : check carefully that the boat structure can handle the significant extra load. you may just add say 100kg, but this is whipping around at the end of a say 2m lever.
do your home work
...employ a professional if necessary...and if in doubt - DONT DO IT
ps : more stability puts extra load on your rig too !