The only way I can currently think of to do this would be to create a calculated field for each Supplier that set it to ZN Sum of Orders. Similar to the following:
if attr([Supplier])='3M' then [ZN Sum of Orders] end
Then, place each of those on the Measure Values/Measure Names card, and put Measure Names in place of Supplier on the Columns shelf. Then, subtotal calculations can be created for whatever Suppliers are wanted.
Note: Since Supplier is an adhoc group, it won't be able to be used in a calculated field. However, a group can be created using a calculated field--http://kb.tableausoftware.com/articles/knowledgebase/creating-groups-using-calculated-fields
Hope this helps!