Hello ,
Is it possile to develop a game using Unity 3D and also some other physics engines?(Bullet, O.D.E.)
Is there a way to integrate them?
with a lot of hackery and drawbacks or if you are a commercially funded studio to license the sources, otherwise no.
PhysX is hard integrated, its not some kind of plugin that you can just replace. The whole collision bases on it too, not just the physics part.
OK, so how can i use the functions from PhysX? i downloaded PhysX_2.8.0_SDK_Core.msi but the functions are not the same.
Also if i want to make a damage model, what is the basic concept behind it? how can i achieve something like that?
P.S.
I am a student not a commercially funded studio 
You can not use any external stuff.
You can only use the phyxs properties exposed in unity, which is basically raycast and all you find in the components - physics area in the editor.
For a damage model you would use basic physics properties from the rigid body and the model class to alter the mesh for the deformation
why couldn’t you use the c# port of bullet for example?
because you will be going to fight against the hardwired physx
its not like there is nothing and you add in physx.
There is physx and you try to bypass it to use an alternative solution.
It might work but it might just as well not work or take a lot of performance (for example, skip the idea of cloth / fluid simulations and alike, it just won’t be fast enough as you have no engine sources to hardwire and optimize the mesh handling for it)