in the current version of the simulator, the precodig matrix is fixed. As you already noticed, it is determined via the scenario file. Currently, only a single user spatial multiplexing transmission mode is implemented. If you want to simulate a MU-MIMO scenario, several changes have to be implemented. First, the scheduler has to be adapted such that more than one user is allocated on the same time and frequency resources. Further, the ZF precoding needs to be implemented in the Links.updateLink routine.
Although there is no multi-user MIMO transmission mode included in the first version of the simulator, we plan to do so in future releases.