I could find only a partial solution for the problem. The following script does the job of the KDE3 one-liner, but unfortunately it still requires a user interaction since plasma doesn't support script executions without a graphical user interface. That's very disappointing.
If you know a way to execute plasma scripts directly please answer this question.
#!/bin/bash
E_BADARGS=65
PLASMA_SCRIPT_FILE=/tmp/kdePlasmaScript.js
DESKTOP_FILE="$1"
PROGRAM_NAME="$2"
PROGRAM_DESCRIPTION="$3"
PROGRAM_EXECUTABLE="$4"
PROGRAM_ICON="$5"
if [ $# -ne 5 ]; then
echo "Usage: `basename $0` <DESKTOP_FILE> <PROGRAM_NAME> <PROGRAM_DESCRIPTION> <PROGRAM_EXECUTABLE> <PROGRAM_ICON>"
exit $E_BADARGS
fi
echo "[Desktop Entry]
Name=$PROGRAM_NAME
GenericName=$PROGRAM_DESCRIPTION
Exec=$PROGRAM_EXECUTABLE
Icon=$PROGRAM_ICON
Path=
Type=Application
" >"$DESKTOP_FILE"
echo "function addStartButton(quicklaunch, panel) {
quicklaunch.writeConfig('launchers', quicklaunch.readConfig('launchers', '') + ',$DESKTOP_FILE')
}
var template = loadTemplate('org.kde.plasma-desktop.findWidgets')
template.findWidgets('quicklaunch', addStartButton)
" >"$PLASMA_SCRIPT_FILE"
qdbus org.kde.plasma-desktop /MainApplication loadScriptInInteractiveConsole "$PLASMA_SCRIPT_FILE"