Hi @uljad
Welcome on the SOFA forum! Always cool to couple physics simulation and 3D printing!
What you will need is to go from a stl mesh to a 3D volumetric mesh (made of tetra or hexa) in order to define on it a proper volumetric mechanical behavior and external forces.
It can be done in SOFA, you can have a look at the scene: examples/Components/misc/MeshTetraStuffing.scn
About starting with SOFA: you can see here a previous post.
I hope this helps.
Best
Hugo