Hi,
Looks like you need to use batch management and maintain the KG value as a batch characteristic.
Your base unit of measure is always piece (PC) and order is created in PC, correct?
You only want to track the weight of each piece and for each lot the weight vary. It's not alternate uom, instead the property of that lot.
So, you can confirm in pc in co11n and produce the stock through mb31, where (if you enable batch in material master), you should enter the Kg for that lot.
Thanks
Prem