Maybe I misunderstand, but I do not think what you ask is possible. There is a plugin to export (backup) libraries, but as you cannot individually import a model including light sources, I do not think that will work.

However, if you have a model with built-in light-source(s), originating from an installed library, in which case you can find it in that library. And with a bit of fiddling (such as deleting other items and merging the stripped library into the one you wish).

If you provide some more detail about exactly what and why you wish to transfer to a library (.sh3f), it may be easier to answer your question.

ok