Fred is quite right that the static objects cannot be turned into playable vessels. They aren't really ships, just a picture of a ship.
In many (most) cases, the AI vessels cannot be used. As described elsewhere, unless the surfaces have been layered in such a way as to be suitably opaqure when viwed from the inside as well as the outside, they simply are not there when you go inside the vessel. And most of them are constructed in that way, of course. Why wouldn't they be? Otherwise it would increase the loading on Ship Simulator to a level where you couldn't play the game at all.
The AI ships are designed to be viewed from a distance. The sailing boat is just a jpg image laid over the top of a box that is shaped like a boat. It's an excellent impression from a short way away, but close up, it does not look so good. So Selsey 1 was originally a blurry impression of edges and surfaces. Although I spent a lot of time adjusting the overlay image, the sailing boat can never look any sharper than it does now or have any more detail in it. But it is a good impression and it is a lot of fun!
The vessels in Ship Simulator 2006 are a completely different format and cannot be transferred across. They need rebuilding from scratch. Although I can do most of that, it also needs a lot of time from Vstep developers to integrate them into the game after that, before I can continue with the other stages. I therefore won't/can't do it. (I want Extremes to come out on time, just like everyone else!).
Similarly, the Voortvarend Besparen vessels are not Ship SImulator 2008 compatible. They are actualy closer to Pro, but not identical. Again, they could be rebuilt from scratch, but...
Now, all that said, there are two special cases that I can add to the game myself. Neither has ever been in Ship Simulator 2006 or 2008. One is very Dutch. The other is bigger than any other ship in the game. I have spent a lot of time making sure it can get into the major ports (Padstow is out, though!). I hope you enjoy both of them.
In both cases, they are nearing completion and are being tested by the team. But something could turn up that means they can't be used. So no promises. Also, no matter what, from the way they were originally developed, these ships can't have smoke or wakes. I can only do that if I seriously disturb Vstep developers and I won't do that.
But they do have walk throughs, detailed interiors and so on.