Alternate answer, since you also asked a more general question.
In general you can add a function call after another function by using advice, specifically
But it's worth noting what the docs have to say in the "Advising Named Functions" section.
advice-add can be useful for altering the behavior of existing calls to an existing function without having to redefine the whole function. However, it can be a source of bugs, since existing callers to the function may assume the old behavior, and work incorrectly when the behavior is changed by advice. Advice can also cause confusion in debugging, if the person doing the debugging does not notice or remember that the function has been modified by advice.
For these reasons, advice should be reserved for the cases where you cannot modify a function's behavior in any other way.
I don't think this is likely to bite you in the specific case you describe, but if you have a recent enough version of Org and can find the
org-log-buffer-setup-hook that I mentioned in my other answer that would be the better solution in this case.