Write to stdin of a running process using pipe
Just ignore the lines containing nc, the OP in this questions wants to use it to transfer data over the network via nc.
That leaves you with:
mkfifo yourfifo
cat > yourfifo &
mypid=$!
yourprogram < yourfifo
Now you can sent data to your program with
echo "Hello World" > yourfifo
If you are done, terminate your program, issue the command kill $mypid to get rid of the dummy process to keep the FIFO open and rm yourfifo to get rid of the named pipe.