and thanks for your question!
The problem you are trying to solve is quite complex, and it is normally a solution our Engineering Services cover.
From Impact testing, you can only derive the local dynamic stiffness at a specific point, but to get the global stiffness of you structure you need more than that. To make it "simple", you would need to measure your FRF matrix, identify high-quality mode shapes (related to high-quality FRFs), and then use the resulting modal model to apply constraints and loads so that you can derive the static bending and torsional stiffness.
Normally, we apply these techniques to estimate the stiffness of car body-in-white, but the same procedure can in principle be applied to every structure.
I attach a paper which was presented at a conference few years ago, which explain the difference between several approaches. I will also send you as private message another paper we presented last year, which follows the same principle but uses more recent tools.
In case you would need more information or support, i would suggest you get in touch with our local office and discuss how we could further support you in solving your problem.