This post answers only partial my question. My problem is that writing to the stdin of the running process using the FD of process on the /proc filesystem does not have the same effect.
start nc to listen on port 10000 (this process is called further nc 1)
nc -l 10000
start another nc to send chars to the listening nc (this will be nc 2)
nc localhost 10000
Write to the stdin on the nc 2
echo "some chars here" >> /proc/[PID-nc-2]/fd/0
the problem: "some chars here" do not get to the listening nc (nc 1), BUT are shown on the console of the nc 2.
Question: why and is it possible to make this working?