I am not absolutely certain, but I suppose that this is the XML file ArchiCAD reads when you click the Material Catalog button in the Fill Types Dialog. So I think there must be one such file.
What I would do is I would create these Material Groups with easily identifiable names. Then I would place them either at the beginning or the end of this XML files. As I see ArchiCAD reads the file contents in sequence so if you put them at the beginning they will be in the beginning in the Material Catalog Dialog.
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