Is it possible to define different regions in STAR-CD where different turbulent models are active, but for the same fluid (material)?

I am thinking of doing a simulation where the flow is turbulent outside of a porous media and laminar inside of the porosity.

Generally the answer is no - you cannot have different turbulence models active in different zones of the same material. But in the specific case where you want a "porous medium" turbulence model and a "non-porous medium" turbulence model, you are lucky as this is what STAR does already. The turbulence model which you specify for the material is never used in the porous medium. The porous medium turbulence model is a simple length-scale/intensity model, see chapter 8 of the Methodology. You could suppress the turbulence in the porous medium using a very small intensity.