If the core command exists in the Default package, then it is possible. However, I have not found a way to change a core command that was not available in the Default package.
I have extended GotoDefinitionCommand to add some custom logic for a plugin I created as an in-house utility. Simply import the class:
from Default.symbol import GotoDefinition
Class CustomGotoDef(GotoDefinition):
...
**** PLEASE USE THIS WITH CAUTION!!! ****
If you need to override the called commands from the menu, make sure your command name Matches the default command name exactly, otherwise sublime will continue to call the original command.