In later versions or MODX, the action no longer has an ID. So editing a resource with the ID of 12, for example, would be:
yoursite.com/manager/?a=resource/update&id=12
"resource/update" is the action.
I'm pretty sure that when MODX sees that, it loads the class file manager/controllers/resource/update and calls its process() method.
One way to do one of your own, would be to put a controller class or php file in manager/controllers and create a tpl for it. Then set the welcome_action System Setting to the name of your controller.
Another way to go: The welcome controller loads the user's default dashboard. So you could create a custom dashboard, with or without custom widget(s) and make sure the appropriate User Groups had that set as their dashboard. The controller gives precedence to the User's Primary User group.
The second method would be much easier.