Standard lengths for purchasing Imperial and Metric bolts are different and you cannot convert and get a match.
You should create separate length Tables for each in PDMS.
The Bolting units is controlled and set up in Isodraft Options file. Ex;
BOLT UNIT METimp
METric
IMPerial
DEFault
Ref. page 2-23 Isodraft Ref. man.
But how can you output the correct term for the metric bolts? I sure want M16. But Isodraft provides me with 16 MM. I haven't found any clues to solving this..