import miscellaneous receipt and generate new invoice #

I'm wanting to import miscellaneous receipts from an excel file and see that the system needs an Invoice # to be included.  When creating a misc receipt manually the system will just assign a "made up" invoice # automatically when the entry is saved.   I guess there's no way to get it to assign the next invoice # automatically for me when I do the import is there?   Will I just have to make them up myself and include them in the import file?