The token passed between pipeline stages can't be inserted from separate thread in the middle of pipeline. The token is passed from first filter to second, than third, etc.
What task are you trying to pass from POSIX thread to pipeline stage? If it is just some data, you can extract and use it in some filter. But this filter should get a token from previous filter, otherwise it will wait for it and will not execute the code taking the data from POSIX thread.
Maybe it's better to focus on initial problem. Can you describe in details what problem do you have with pipeline stages? Code sample would be very appreciated.