First off this is for in-game options, and also this is for the basic menu.
/------Starting off------\
1) Open the script editor.
2) Go to Scene_Menu.
3) Go down until you see lines 54-60.
4) make a new line after line 59 and add s7 = "Custom" <-- you can change it.
Also add s7 to the current line 60.
/------Making it work------\
1) Go to line 90-97
2) Add a line after line 97 and put
when 6
$scene = Scene_ And put what ever you want to open when you use the new option.
an example would be
when 6 # Achievements
$scene = Achievements.new
If you did everything correctly it should look like this: