Nice work. But here's few things and I believe that's all for pocket expansion.
1. You need to add case to in function to map the use item action message box frame. (Just duplicate ITEMS_POCKET case since the new pocket exactly same actions.)
2. You shall also consider LoadSave action of the new item slots.
All right, I did the first one, but I'm not sure what you mean by the 2nd one ^^; (I've barely started playing with pokeemerald, so I'm not too familiar with it.