why the unit of measure interclass conversion is restricted to Base UOM

Summary:

In this UI - there are two issues.

1. After selecting item, the "to Base UOM" cannot be edited. In my business case - the item UOM is KG and the base UOM is Pb.
2. The from UOM Class is also not editable for the user
3. The from UOM code is restricted to base UOM of all the UOM classes and not to other UOMs.

I need to define item specific UOM conversion between KG and Gallon and with this approach it needs three UOM conversions( ounze to Pb, Pb to KG, Gallon to ounze) which is very time consuming and inefficient for the user and is very much resented