A tool bar has a reference to a single image list. This is a property of the underlying Windows control that is simply reflected on to you by the VCL.
The conclusion is that you must put all the images into the same image list. You cannot have two different image lists referenced by a single tool bar.
One solution would to pass your image list to the plugin and allow it to add glyphs to your image list.
Note that I'm talking about the primary image list. Naturally you use different image lists for disabled images and the hot images. But those don't really contain different glyphs–they contain the same glyphs but rendered in different states.