If i understand your problem correctly, you need to calculate the pressure resulting from the impact event then apply it on the structure. You can calculate the pressure exerted using the empirical correlations available in the literature.

BTW, which tool you are using for topology optimization?

if you are licensed user of Abaqus, then in the recent release, you can perform topology optimization using Abaqus/CAE or TOSCA. Hope you are already aware of this :-) 

Thanks Ranjith. I am already using Tosca. So after several attempts, conclusion is that ESLM is probably not a feasible solution for this analysis. The results would have to be validated after each iteration and each run takes quite a bit of time if solved implicitly. Parametric-based optimization approach using heuristic algorithms would work better for this.