you create a characteristic "submaterial" and allow multiple value assignment
you create a class in CL03, for class type 001 and assign the characteristic to it.
you maintain your material master classification view and add the class.
and here you add all your material numbers for submaterials to the characteristic
follow the steps in How to Assign an UNSPSC Code into Material Master in SAP without using any Enhancement
and Cook Book - Classes and Characteristics - Supply Chain Management (SCM) - SCN Wiki
