The Budget Account Entry data entity includes the quantity field.  This field can be exported using data management and can be mapped for import, but it is ignored during import.  

We consider this a defect, but Microsoft closed our ticket claiming that it was as designed and we would have to request a customization from our partner.  It should be noted that quantity is not disabled for the general journal import which closely corresponds to the budget account entry, This is an oversight and thus a defect.