I would like to point out that converting RGB to CMYK (or vice versa) is rarely that straightforward. You need to ask “which RGB” and “which CMYK” for there are many.

My guess is that the best solution would be to export an RGB CMYK and then use Acrobat to convert it to CMYK.
Or use InDesign’s built-in conversion tools to export a CMYK PDF, as we discuss here:

Why You Should Import RGB Images Into InDesign and Convert to CMYK On Export

We have a few scripters who read here sometimes, but you might get even more help at the InDesign Scripting group on FB: https://www.facebook.com/groups/indesignscripting

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