See also the related article about units: How-To: Retrieve the corresponding unit of a PRC model
How can I overwrite units inside of a PRC file ?
The easiest and fastest way to proceed is to create a new modelFile (with the A3DAsmModelFileCreate function) or to edit the root PO (you can use A3DAsmProductOccurrenceEdit for example), by setting m_bUnitFromCAD to true and put the desired m_dUnit (defined as a mm multiple).