It's perfectly possible to replace a player or hostage model from CS with your model. However, you'll have to give it the same animations. I'd advise to use the skeleton of a CS player model and attach your vertexes to that skeleton. The mesh can be changed without having to reanimate the model, but once the skeleton is changed, all animations have to be made again. And believe me, that's not what you want to do...
Just place your model, once it's ready for use, in the models folder and give it the same name as the model you want to replace. You may want to backup that original model, bytheway...