Would you inform me does Star-CD models the fan? Say, for the problem of incompressible flow in a duct with fan. (In Fluent there is internal boundary condition named "fan")

You can use a body force fan, rotating reference frame, or sliding mesh. Which one you choose depends mostly on the number of blades on the fan.

It sounds like you want a body force fan, which is simply a momentum source which can be added in the star guide.

Better off using Star-CCM+, its a lot easier to do and you can easily use a fan curve to specify the source.