How to add custom locale to /etc/locale.gen

[gwmfms6]

I have placed a custom locale in my /usr/share/i18n/locales folder 
(e.g., en_US-cus). But when I go to vi /etc/locale.gen to uncomment it 
it doesn't show up.

How can I get it to show up so that I can generate it with sudo 
locale-gen?