Since this is a Zemax question, it is better to post in Zemax community:
https://community.zemax.com/
I found some information here
https://support.zemax.com/hc/en-us/articles/1500005491181-How-to-design-DOE-lens-or-metalens-in-OpticStudio
Since usually the metalens is designed with a plane wave illumination, so when it is the first element in the optical system, the imported FDTD result as the source has the information of phase (and thus the wavefront). with the help of POP, it propagates to the conventional lens so the rest is easy.
If the metalens is not illiminated with plane wave (such as it is inside the optical system), things can be complecated. This is because it can be a challenge to design the metalens with proper phase profile required, since the elementary composites (eg, rectangles, cyilinders or rods) might not be simulated with periodic boundary conditions, due to the fact that the illumination is not a plane wave. It might work but will need special considerations. If this is the case please try to find some references and see how others do it.
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