Hi Rurangwa,
Could you be a little more specific and give us an example? It is of course not allowed to insert or alter data directly in the database, but you could use other tools like Formatted Search, ex: to fill the duedate with the Postingdate + 30 days in an invoice you could use:
SELECT CONVERT(DATETIME,$[OINV.DocDate],5)+30
Let me know what it is you are looking for.
Kind regards,
Andy Grootens
Asecom