The sub menu you want to add the function to, just copy one of the menu options that exist already, and paste it underneath it like so:
self MenuOption("Bullets List", 11, "Hanter Killer", ::selectMB
self MenuOption("Bullets List", 11, "Hanter Killer", ::selectMB
Note that each menu has different codes for the menu options. They could be self MenuOption like this, or different like self.add_option.
But when you have done this, you can rename it to whatever you want. For example, Ill name it God Mode! This is the text that will be visible when you are in the menu point of view. Also, if your option has a number, make sure to rename it corresponding with place it is at.
self MenuOption("Bullets List", [B]12[/B], "[B]GodMode[/B]", ::selectMB
You also need the function for whatever you are adding in. As I a adding in god mode, you need to find a place to store the function. I always add it at the bottom to the menu so If I screw up and get script errors, I can easily find it and remove it.
And btw, the function should look some what like this:
Toggle_Godhost()
{
if (self.GodMode == false)
{
self iPrintln("GodMode: ^2Enabled");
self.maxhealth=999999999;
self.health=self.maxhealth;
if (self.health < self.maxhealth)
self.health = self.maxhealth;
self EnableInvulnerability();
self.GodMode=true;
}
else
{
self iPrintln("GodMode: ^1Disabled");
self.maxhealth=100;
self.health=self.maxhealth;
self DisableInvulnerability();
self.GodMode=false;
}
}
Anyway, when you have the function added. You can then add the function to your menu option.
self MenuOption("Bullets List", 12, "GodMode", ::[B]Toggle_Godhost[/B]
Hope this helped and good luck on adding the custom options to the menu 
Any other people reading, feel free to correct any mistakes I may have made. I am not the greatest of coders out there.