By default, you'll find the Extend function below the Record Monitor and in the Fast Menu Tool Palette. It can also be mapped from the Command Palette (Tools menu | Command Palette | Trim tab). The icon looks like the following:
In many instances Media Composer offers two or more methods for getting something done. This provides you with choices, depending on your preference or the needs of a particular situation. This is the case with Extend. The end result of using the Extend function is identical to using Double Roller Trim or Overwrite Trim. One of the benefits of using Extend is that if it's mapped to the keyboard, then all the operations required to use it can be done using the keyboard without even entering Trim Mode.