You can use the FieldNumber function to determine whether a field exists in a table. For example FieldNumber('MyField', 'MyTable') returns the position of MyField within MyTable.
If MyField does not exist in MyTable the function returns zero. You can adapt this to your advantage by loading all fields from your QVD into a temporary table and then checking to see if this table contains the field. If it does, you can continue loading. If not, you can simply set the field to zero.