Create combined URDF model

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What is the best way to create a model that combines a mobile robot (e.g. pioneer3) on which is mounted a laser scanner (sick lms200) and a robotic arm? Should I out all the urdf descriptions in a single file and connect the objects with a fixed link or there is a better solution?

I have the complete model (including urdf, mashes, xacro) of the mobile base and the arm, but for the scanner I have only the *.urdf.xacro and *.dae files

Update

The scanner urdf can be transformd by: rosrun xacro xacro sicklms200.urdf.xacro > sicklms200.urdf