I made a script for that, but since you are asking for support I won't pos it here :P
If you go in Scene_Menu, at line 53 you will find the create_command_window_method. In there, there's a couple of variable representing the menu commands. Then, there's this line:
@command_window = Window_Command.new(160, [s1, s2, s3, s4, s5, s6])
It creates the command window with the 6 default menu commands.
The first thing you need to do is to create another variable, like
and change the line where the window is created for this:
@command_window = Window_Command.new(160, [s1, s2, s3, s4, s7, s5, s6])
Now, you will have the Job command in the main menu. But you're not done yet. This command will bring you to the save screen because it kinda took the place of the save command.
Next step, go to the update_command_selection method and change this
case @command_window.index
when 0Â Â Â # Item
  $scene = Scene_Item.new
when 1,2,3Â # Skill, equipment, status
  start_actor_selection
when 4Â Â Â # Save
  $scene = Scene_File.new(true, false, false)
when 5Â Â Â # End Game
  $scene = Scene_End.new
end
for this
case @command_window.index
when 0Â Â Â # Item
  $scene = Scene_Item.new
when 1,2,3Â # Skill, equipment, status
  start_actor_selection
when 4Â Â Â # Job
  # your code here
when 5Â Â Â # Save
  $scene = Scene_File.new(true, false, false)
when 6Â Â Â # End Game
  $scene = Scene_End.new
end
Notice "when 4", the original index of the save command was 4 and so, when the cursor is at index 4, it will open the save menu. Now, there's no code there. You'll have to put your own code to open your Job scene.
Hope it helps!
-Dargor