Is it possible throught scripts or else change the race of the player ?

and on top of that make a custom race or kind of similar to the Vampire?

Let's say I want to make a new ilness that tanfoms you into some kind of a monster ( like for example happens for the vampire ) but I want to use a new custom racial feature , woudl that be possible? and what woudl I need to model , texture or create in order to make that happen?

I know that people have done this and that it is doable. However, I personally have no idea how to do it. I'll be interested to see if you can figure this out. If you do, post your findings. I would like to know how to do it myself.

Yes, yes, and yes.
New Races involve more than just the race item though, check out an armor called skin naked and the "armor addons" it uses to give the body of te new race some substance.

A snippet from a script would look like this:

akActor.SetRace(MyRace)

You do not want an illness to take hold of a vampire or a werewolf without accounting for them being Special Case Races though, which can be tricky.