Here is a tutorial for impact testing:
To measure dynamic stiffness (Force over Displacement), follow the tutorial and add one additional step:
When in either the Impact Setup worksheet or the Measure worksheet, click on "All settings" in the upper right corner. In the 'All settings' menu, turn on 'Dynamic Stiffness' under data storage. Picture below:
Do this before taking the measurements. In addition to a Acceleration over Force FRF, a dynamic stiffness FRF will also be measured at the same time.
If you have tested the FRF, then you can double integrated the FRF and Inverse it on the Data Calculator of the Navigator Sheet. Then you can obtain the dynamic stiffness.