The best (and maybe only) way I've figured out how to do this is a bit of a hacky workaround. But what I'll do is add all of the billable expenses to an invoice, then proceed to delete them all choosing "No, keep it" when asked "Would you also like to unlink the billable expense?". This way, you have all of the underlying vendor invoices linked up, but you can enter your own custom line items. Not super pretty, but it gets the job done for us.