If you don't need the "tracking" provided by using a PO, just enter them as vouchers with the voucher # being the invoice number.  Enter the correct invoice date for aging.  You'll need to know what GL account you want them to post to.  If they need to go into inventory, then it gets a little messy.  You'll have to make an inventory adjustment, but watch what accounts are actually being  
posted to between the voucher and the inventory adjustment...it may require an offsetting GJ entry.  Same type of issue if they need to go to job cost/work orders.  In both those cases, I'd choose to enter a PO, receive it, etc.
 
If you need PO info, you could just enter a PO & receive it etc.