How to enter due date directly on invoice without defining payment term

All,

We have a peculiar situation here. We import AR invoices from other source. To the most of invoices we have standard payment terms and importing invoices are not a problem.

But few of the invoices the due date is determined case by case basis as per negotiation with customer. Its difficult to define payment terms for each of the case as the information is not available up front.

Can we enter due date directly and NOT have to enter payment terms?

If you can suggest a way out it will certainly help us. Appreciate your help.

Thanks.

Anil