Hi,
As per my understanding you have base unit in KG and you want to material issue and sales in miligram unit. If this is requirement then you may try to assign "sales unit" for material master in "sales:sales org1" view. you have to also give the conversion rate from KG to miligram in additional data.
for issuing you have to maintain issue unit "miligram" in material master view "Plant data/stor1"
Regards,
mukesh