Hi Priya,
It is not possible to change standard program without 'access key'. Access key is not for security purpose but it is for licence.
If you are adding new field to standard program, you can go for enhancement by using exit's where you can add you own code.
If you want to try with enhancement you can refer this below link:
http://wiki.sdn.sap.com/wiki/display/ABAP/Enhancements+(User+Exits+and+Customer+exits)