I need to have a DC line
Maybe I simulate this DC as a ground line. I am not sure about that
You don't wnat to hear this, but the much better layout is to place your DC line away from the RF line !!!! If it is outside the main RF field, shielded by the side grounds, it will be much less trouble.
In your configuration, with the "DC line" inside the CPW, much RF will couple into the "DC line". You can tell the line that it's a DC line, but that is just a
name, and doesn't change the physics of coupled lines. Your "DC line"
will carry RF signal and you will have trouble with RF leaking around in your cuircuit, distributed via the "DC line" that really has DC + coupled RF.
But if you still want to simulate with the "DC" line in this place, treat it like an RF line. This is the ONLY way to get useful results, because you WILL have coupling between the lines. ONLY if you KNOW the RF impedances at both ends of the DC line, you can use that, otherwise use ports and then check realistic terminations of the "DC line" in postprocessing.