I can think of one rather complex way to do the co-simulation Aasif, and it would work only if co-simulation of OptiFDTD and Matlab is possible. You can use Matlab component in your design, and in the corresponding Matlab program you can call OptiFDTD to do the calculations and give the results back to Matlab. This way, results can be used in OptiSystem using two interfaces.
The above method only works if Matlab-OptiFDTD cosimulation is possible. However, I thought it was worth mentioning as it may work.
Regards