The by far easiest is to add the model and the light source separately (group them if you wish to move model and source).

If you do not feel daunted by diving somewhat deeper, you can edit the properties of library models yourself.

the SVG guide, pages 9 and 10 gives you an example of how to edit/add the properties, and also light source properties, in the library file. That is just an example, but in the links you should find more information about editing properties.

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