Bill Import - How to populate the Reference Number field

Bill Import - How to populate the Reference Number field

Hello,
I am importing a large number of bills from a CSV file. All the fields are mapping nicely but I cannot find how to add something to the Reference Number field. Can anyone help?

It simply doesn't show in the list of fields on the import screen. Is this field accessible somehow? I would really like to use it because their is a Reference Number column on the list view in Bills and would be a handy way to quickly check certain info without opening every individual Bill.
Emma