how to define urdf content when robot has rack-gear frame?

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I try to make a robot with dynamixel and some kinds of frame which i made.

this robot is some special because I use rack-gear frame and pinion gear in my robot. Like this one :

I want to use robot_state_publisher and It use kdl_paser using urdf.

My problem is I don't know How I make urdf file. normally, robot actuators are connect directly using joint or frame. and I already did urdf tutorial, and make some robot arms like open maniulator.

but if world base have rack gear and link1 moves on rack gear, I don't know to define urdf file.

Could I get a solution about this problem? or get a tutorial or insight?