Ok I thought I'd answer this one just to help out. You're making calls to another computer so I'm assuming that you are reading in streams of data in char*'s. That's what I get from a comment like "I am getting the data in a char* object in parts."
Therefore, as I understand it, you just want to append the char* string to the IStream object so that it holds the new data.
My suggestion would be to do this:
Create a streambuf object (http://msdn.microsoft.com/en-us/library/aa277891(VS.60).aspx)
While you are recieving packets
append the packet to the streambuf object (http://msdn.microsoft.com/en-us/library/aa277883(VS.60).aspx)
Create a IStream object and init it with the streambuf object (http://msdn.microsoft.com/en-us/library/aa277365(VS.60).aspx)
Now that should work I think. That will fill your IStream object with the stream you got from the website.
I hope that helps,
Robert