This is typical question How to add field in Onhand Inventory form in Ax 2012 but I got easy way to solve. Only drag drop field will not help on this query .
You need to reach on class InventDimCtrl_Frm_OnHand then find method modifyQuery then you need to add your field in group by otherwise you never be able to add field in on hand form. I hope it will help you a lot
You should write code at the Classes\InventDimCtrl_Frm_OnHand\modifyQuery() method This is called when you open the On Hand form, where Item number is shown and group by with other field.