Can CST or XFDTD or any other FDTD simulator model and simulate Electromagnetic Pulses (EMP) (It has a very high power and very short period - femtoseconds- .
If you use CST, you can write your own excitation function macro code.
Go to "Transient Solver"->"Stimulation Signal"->"User Defined",
a macro editor should pop up. This editor contains a test function that plots your function on the defined time scale.
If you have any trouble, there is a simple sine function example in CST help documents.