SAP ACDOCA have 3 fields for Quantity and unit of measure UOM. which can be configured to be updated from SPRO Controlling, once the set config is done you can implement the BADI FCO_COEP_QUANTITY in the mentioned BADI there is an method with name CALCULATE_QUANTITES there you can write your code to convert one UOM to another.
There is a table T006 which will also help u know the conversion of UOM to another is possible or not. Field DIMID of the UOM should be same to convert the data.
Function Module UNIT_CONVERSION_SIMPLE help you to convert one UOM to another.