No, there is not. And if there was a way, setting the NumBytesAvailable would confuse the system because it would either lose track of data in the buffer (if you are reducing NumBytesAvailable) or would not have data in the buffer available (if you are increasing NumBytesAvailable)
If your purpose is to sort of reduce NumBytesAvailable to imitate a smaller input buffer, then at the point that data is to be read, ask it to read the minimum of NumBytesAvailable and the desired buffer size.
