Dear Rohit,
The definition for 'agent' itself clear points that those are the only options available:
Agent
A chemical agent or physical agent that can cause harmful effects in an exposed person.
I am not sure why are you expecting to create a custom agent type in this scenario since
physical agents (such as noise) and chemical agents (such as acetone) would be enough to cover all
related information.
It's also described in the field help the following:
Agents
Specifies whether hazards assigned to the hazard category can involve chemical agents,
physical agents, or no agents at all.
There's no function to assign any other type in this scenario.
I believe you may also check the following documentation :
http://help.sap.com/saphelp_ehsext30/helpdata/en/07/2c1e286bd44fde8df5f06866587318/content.htm
and ...
http://help.sap.com/saphelp_ehsext30/helpdata/en/23/b20f43c013494b8ed74603dec667a4/content.htm
Hope it clarifies!
Kind regards,
Raphael