I remember some users asking about this in the past. In QTP or UFT, if a user performs a "Call to Existing Action..." within a GUI test, the external action is opened as read-only, so many may ask to themselves "is is possible to edit that external action from within the local test?".
In answer to that question, inserting a call to an existing action will let the user view the steps of the action in the action view, but it cannot be modified. The called action's local object repository and the action's name will also be read-only.
To modify a called external action (including changing the action's name, Object Repository and action steps), it is necessary to open the test with which the action is stored and make any modifications there. The modifications apply to all tests that call that action.
If it is not possible to open the test that contains the external action, and modifying the external action is necessary, the user has the option to add a "Call to Copy of Action..." instead, which will create a local copy of the external action in its entirety. Any modifications performed to this copied action will not be reflected on the external action, and will only be applied on the local test.
I hope this information helps anyone out there.