Because programming something you can easily create yourself does not make sense?
Create an action to convert RGB to CMYK and then use that action in batch conversion.
Automate batch dialog is used to batch an Action. Could it be that your using an action that has a recorded Automate Batch step and that action or action set recorded into that batch step is not currently in you actions palette?
I think all a convert to CMYK action would need to be is a single insert menu item "Image>Mode>CMYK Color".
It is it sensible to batch something that is easy to do when you have many files you need converted..