UCUM units of measurement in Archetype Editor

Archetype Editor let’s to specify the list of units of measurement for a DV_QUANTITY element (C_DV_QUANTITY constraint). Not all the UCUM units of measurement are selectable from Archetype Editor. For example [MET] (metabolic equivalent) is defined in UCUM specs (http://aurora.regenstrief.org/~ucum/ucum.html#para-44) but I didn’t find it in Archetype Editor.
Using Archetype Editor, how can I define a DV_QUANTITY as [MET]?

Thanks
leonardo