Hi Murtaza,
I don't think you will be able to cater your requirement through standard customising.
You have to enhance event 1330 for the same.
Event 1330 gives you the option of determining the document due date from the payment conditions when posting a document. So you can code in your conditions for the particular payment terms in event 1330 and it will do the rest.
Hope it helps..
Thanks,
Amlan