Hi:
I don't believe that Input Streams were meant to populate a parameter list or if they even COULD populate a parameter list. Since the constraints for the parameter list lives in the metadata, it sounds like you want the parameter list to be dynamically updated "on the fly" from your stream. I am not sure that you can do this with streamed input to a stored process.
As an example of using stream input, in this Stored Process, the data from the stream (INSTREAM libref) is being sent to PROC MEANS:
http://support.sas.com/rnd/itech/doc9/dev_guide/websrvcs/sample.html
This is a question that is probably best answered by Tech Support.
cynthia