CST will do it - THz radiation is no different to other frequencies. You will need to know the properties of the materials you use, at the frequencies you choose. Your problem size in wavelengths will be limited by memory and speed just as for other frequencies.
The same applies for most other prediction software.
If you are talking about THz radiation (EM waves) then obviously yes. Like any other part of the EM spectrum. But in case you are talking about generation from different sources (like plasma, solid state, photo-conductive, photo-mixing) it is not possible at all. These processes require Multiphysics software (Like COMSOL), then also direct radiation output is not likely but some degree of simulation is possible. In present scenario simulation in these fields are mostly analytical in nature..