This is on the 'to do' list, but there is partial support already (which might or might not be sufficient for you). If you are comfortable with updating the database directly (jos_nbill_product_price), all prices are held in 6 decimal places. Even so, some calculations might still presume 2 decimal places at the moment, as I haven't implemented it throughout. You can also amend the /administrator/components/com_nbill/framework/nbill.number.format.php file and change the default decimal places there (although that might cause too many decimal places to appear in some areas).
