The Exp Share (from Gens 2-5) is a separate item, and was coded first, so it got the internal name EXPSHARE. What you want is an item we'll call the Exp All, since it's similar to the Gen 1 item of that name. The On and Off states of this item are actually two separate items, so the two internal names you need for them are EXPALL and EXPALLOFF. Effects already exist for items with these internal names.
Remember that the name of an item doesn't have to be identical to its internal name. You could call it "Sausage" and still give it the internal name EXPALL.