OutSystems Service Studio already allows adding to the Built-In Functions by creating folders with matching names. For example, creating an Action Folder named "Environment" with a function "GetEnvironmentName()" adds this function to the Built-In Functions.
However, there is currently no way to add to Built-In Functions with spaces, such as "Date and Time".
This could be fixed by matching with folders that have underscores, such as a folder "Date_and_Time" with a Function "GetCurrentTimezone()".