Right, you could replace a part with a spring of a proper (calculated) stiffness in each direction. Of course, you can attach it via rigid body constraint but distributing coupling constraints might be better (not overstiffening the model). Unfortunately, they aren’t supported in PrePoMax yet so you would have define them manually with Keyword Editor: RBE3 integration in PrepoMax (Kinematic & Distributing Coupling)
Coupling constraints in CalculiX have certain important limitations: Different coupling constraints and their limitations - CalculiX (official versions are on www.calculix.de, the official GitHub repository is at https://github.com/Dhondtguido/CalculiX).
In this example (dynamic analysis), spring is attached via rigid body constraint: https://www.youtube.com/watch?v=R8HQKI3PTrI