Hi Michael,
maybe I'm doing something wrong, but the select influences option is available when you merge something. As you can see from the screenshot attached, I basically select the objects I want to transform into a xref scene, click on convert selected, which opens a path saving dialog, and save them where I want them. When the saving is done, some of the forest pack objects are gone, both in preview and in render.
Please note that the forest pack objects are coming from a preset I made, so they are basically saved on another file.
Regards,
Hadi