It depends on how much the rebar influences the overall mass (M) and the structural stiffness (K) in the equation f1=(K/M)^2 and how complicated the geometry is. The stiffness is a function of the modulus. The mass is a function of the volume and density.
If the geometry is simple enough, perhaps you can calculate an effective modulus and equivalent density to represent the composite of concrete and rebar so you do not have to include the discrete rebar in the model.
If you are doing this for first time, you should build a small simplified representation (with and without the rebar) to validate the assumptions.