How to create USD currency under "Currency" hierarchy?

Hello everyone,

I am using PBCS, and my application has the base currency "THB".

Now, under my "Input Currencies" hierarchy, has only 3 currencies :

I tried to create USD currency and its reporting currency.

However, I could not find it under "Standard Currecies" code:

My question is:

How can I create USD currency ? Did I miss any steps?

Thanks!