Hi,
In the core of HikaShop, HikaShop skips the payment methods when the total amount is 0. So changing that would require some core modifications, which I don't recommend.
If you only use the purchase order plugin, that means that you don't have any payment on your shop.
In that case, why not use instead the "collect on delivery" plugin which is the same as the purchase order plugin but without the PO number field, and create a custom field of the table "order" via the menu Display>Custom fields in order to collect the PO number ?