How to Create a Custom HTTP Error Response using AppCmd

I am trying to create a Custom HTTP Error Response using AppCmd as explained in 

http://technet.microsoft.com/en-us/library/cc753103(v=ws.10).aspx

The only difference is that I want to create custom error response for only one particluar virtual directory. 

To make it clear, I have a Web Site under IIS named 'MyHost'. My host has multiple virtual directories under it have different applications.

What I want to do is create a custom error response for only one virtual directory.

Ofcourse, i can do it using the standard User Interface but what I see is that IIS7 modifies my web.config which I do not want due to some old code and dependecies.

Thus, I want to modify the ApplicationHost.config itself and put the <httError> section using AppCmd.

Or if there is any other better way, please suggest.

Thanks in advance.