Hi Roopesh,
You can create an Extension Field for class Contract Document and using page customization you can place it in between field Description and Contract Document type.
you can get to know how to create Extension Definition from here
http://help.sap.com/saphelp_sourcing_70_p/helpdata/EN/fb/6026cc66f94b018a5902d7ebc58ce0/frameset.htm
To know about page customization please refer
http://help.sap.com/saphelp_sourcing_70_p/helpdata/EN/62/a8ca4e7504439e8f247efc11d9a36e/frameset.htm
Hope it helps.
Regards,
Kumud