Depending on the version of SketchUp that you have installed on the client machine, you will have the option to export to a number of supported inter-operable formats. For instance, SketchUp 2015 will allow you to export to the following formats:
Even older versions such as SketchUp 8 will allow you to export to the COLLADA XML (.DAE) standard which is one of the most widely used 3D modeling formats available. All versions of CityEngine will have direct import capabilities for that format. This will also ensure that the models can be utilized in other modeling platforms such as 3D Studio Max or Blender.
Just want to link the documentation for 3D formats you can import into CE (see supported formats for Static models).
Generally, I have had the best experience with clean OBJ files (in the sense that you don't have texture conflicts or issues with normals that carry over from sketch up.