Its a lot of work so its not surprising RFB never finished it. You have to individually mod each ship class upping the hitpoints and adding new zones, using the S3D editor. Some ships, like the IJN fleet carriers, are notoriously easy to sink in stock.
The stock sinking model is not just hitpoints, it is a combination of hitpoints and flooding. The problem is that ship movement is tied to hitpoints, i.e. a ship will only stop moving when it loses all its hitpoints and the game knows it is definitely sinking. The problem with the pure flooding approach is that the ship can keep on moving even though it is heavily listing or even partly underwater.
TMO also uses the stock sinking model. I imported the RFB model into it, but was not happy with the result. I had done a mod which leads to more acceptable results in TMO IMHO, i.e.:
http://www.subsim.com/radioroom/showthread.php?t=193610
It would be possible to adapt it to RFB so the other warships sink at a slower rate. However, you cannot just dump it into RFB since RFB heavily customized the Zone cfg file as I recall. Someone would have to edit the appropriate zones in the RFB Zone.cfg.