Just extend the filter:
where (“Item Category Code” = filter (‘ROLL STOCK|SHEET STOCK’))
Although, I would suggest to avoid hardcoding filter values in table relations - these values tend to change with time. Much better if the filtering list is stored as a setup in a table and applied in the OnLookup trigger. Of course, setting a table relation is easier that handling it in code, but its simplicity should not lure developers into bad coding practices.