RLCG parameters are defined for a homogeneous line segment. I would first calculate the line parameters (line impedance and effective epsilon) from the RLCG parameters, and then calculate the S-parameters from there.
I understand that you have RLCG and want the S-parameters.
In the link that I have you, you can find the math that relates RCLG and gamma and S-parameters.
Take RLCG, use quations 5 and 6 and use these values in equation 2, then you have the S-parameters.