convert a .s2p file to spice model
In general, there is no easy way to do this. The best you can do is guess what circuit might produce those S parameters, then adjust the circuit parameters until you get a good match (possibly using optimization software). 

The more you know about the actual details of the circuit, or the construction of the device you're modeling and its likely parasitics, the easier it will be to do this. 

If the circuit you're modelling is a single transistor, there may be parameter extraction software available to do it. But you'd need more than just one S2P file. Something like S2P files at at least 2 bias points, plus dc curve traces, is likely to be required to get any kind of useful model.

If you use ADS rather than SPICE, you can simply tell the simulator to use the S2P file in a "data driven device model" to simulate the device. Of course this will only give accurate results if you bias the device at the same point that the actual device was biased when measuring the S-parameters, and it won't be useful for DC analysis.