You cannot do that at the present time. Nor is there a strong reason to import many sales invoices in most cases. If you are just beginning to use Manager, the only sales invoices you need to recreate are those that are still unpaid. Doing this will establish your Accounts receivable balance. If you are converting from another accounting system, however, you will want to set a Start Date. Be sure to list the actual issue date for those sales invoices, even though it is before the start date. Other accounts requiring an opening balance provide an opportunity to set the balance as of the Start Date.
One reason you cannot import sales invoices in bulk is that for each invoice you must first create the customer. So it cannot be a one-step process.