Allow to use custom "order by" in product category

In magento can define the "order by" for each product category, but when import product categories in OpenERP it crash.

"order by" now is a field.selection, and I need to add to this selection custom "order by".

I need to create new object for customs order by and change field.selection to fields.many2one.

Then if import product categories crash I could create new "short by".

I am developing these feature in a path module, if blueprint is acepted I will do a merge proposal.