The Attribute manager enables import of specific materials_attributes_from any file - your specific client PLN reference file, for instance - easy if you prefix material names with the client's name or ID.
Another way is to make a set of simple objects - a matrix of slabs, say - with all of the client's materials included on them. Copy these carrier elements from one open ArchiCAD file to another - the process also transfers the material definitions - you can immediately delete the objects and the new materials remain on the list.
Dwight Atkinson