Is there any way like without modifying my SNMP Agent code, i just modify the MIB (like adding some parameter or variables into it) & from that i'll be able to send trap with modified MIB without doing any SNMP Agent code change?
What you want cannot be achieved, as MIB documents are only a contract between the agent and the managers. When you receive a MIB document, the agent has already implemented it, and even if you modify the document that contract inside the agent never changes.
I would be surprised if you were the developer of that agent, as in that case you should have known how a MIB document is used during the agent development, and you should know modifying it alone won't affect the agent.