Those fields are for the conversion between your current unit and the SI unit of the same dimension. SAP has a pretty good OSS note for units and conversions 362932 - Conversion with proportion/product units
This note alone should give you an impression about the complexity. Quantity and value domains would certainly be the very last thing to change in a SAP system, the risk of field overflows is just too big.
I agree with Tom, if KG is not accurate enough then use Gram instead.