Poser, as far as I know, will only save props in the Prop library, lights in the Light library and so on.

I think the only way you can do this is to save as a PZ3, edit that file using a text editor, then rename to PZ2.

The best way to see exactly what you need to have in the file, would be to save the lights and prop seperately (to be used for comparison), then when editing the PZ3 you need only what is in the LT2 and PP2 files (the difference will be that the PZ3 will have the parenting information).

I think there are some light sets in the FreeStuff that use this technique, so examine them to see how it's done.