Hi Sanjeev.Kejriwal,
You can not convert string to System.Web.HttpContext.
Based on my understanding, I think you want to add an item to System.Web.HttpContext.
You can use the following code to add value:
Context.Items["oneItem"] = "add one item value";
The static property Current on the HttpContext class can be useful whenever the flow of control leaves the code in your Page derived web form. Using this property you can reach out and magically grab the current Request, Response, Session, and Application objects (and more) for the request you are servicing
For more information which is related to the System.Web.HttpContext, please check the following links:
Working With HttpContext.Current
http://odetocode.com/Articles/112.aspx
http://www.odetocode.com/Articles/111.aspx
http://aspnet.4guysfromrolla.com/articles/060904-1.aspx
http://msdn.microsoft.com/en-us/library/system.web.httpcontext.current.aspx
http://msdn.microsoft.com/en-us/library/system.web.httpcontext.aspx
I hope it is helpful to you.