send command to an already running screen session

Actually it's quite simple. You can use the following:

screen -S sessionname -X stuff 'command'`echo -ne '\015'`

echo -ne '\015' emulates pressing the Enter key.

NOTE: that 'stuff' is a screen command. http://www.gnu.org/software/screen/manual/screen.html#Stuff

Sounds like you want:

$ screen -S test -d -m -c /dev/null -- sh -c 'date; exec $SHELL'

-S test: name this session
-d -m: start screen detached
-c /dev/null: ignore ~/.screenrc (provide a different file or drop this option,
  as needed)
--: end of screen options
sh -c 'date; exec $SHELL': command for screen to run, note that if this command
  was just 'date' then screen would exit immediately.  if you don't want to exit
  screen after your command exits, then you might use:
    $ screen -S test -d -m -c /dev/null -- your command here

Or maybe you just want dtach.

screen -S <session_name> -X screen [-t <title>] <shell_command> [<args>]

Open a new window (with title title) in an existing session named session_name, and execute shell_command (with optional arguments args)