How to add Cost element(material overhead) for existed items

Hi,

We have reqeuriment to capture Materila OverHead in our business though we have never used so far this element in our business.

However when i creater new item code and do the PO,WIP transactions it has captured material overhead cost as i have setup the material overhead and absorption rules and defaults etc..

But for existed item codes even when i run the "Mass Edit Material Overhead Costs" oracle does not capture the material overhead cost untill i add this element to item cost manually. Also, above request does allow to choose cost type - Avg rate . Our main Cost type - Average.