We have exmaples for phase masks (you can search online) however it seems you want to use a phase mask to create some special light beam with a given input light beam. right? if so I think you can simulate, just try. FDTD solves Maxwell's Equations so as long as the mechanisim of creating such beam can be described by Maxwell's Equations it should be able to simulate. The question might be how to quantify the created beam, or how to justify it is the beam you desire.
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