Convert graphics from RGB to CMYK
I use InDesign for a lot of print layout, and often graphics used/supplied (stock, photographs, etc.) are in RGB format. But for print I need these in CMYK format, and since ideas for which graphics to use and layouts change often before finalization I wait until the final OKs are given before converting graphics to CMYK.
The way I see this working is through the "Links" tab, although perhaps it could be done through the "Package..." function. In the Links tab, it would give me the option of right-clicking an RGB file and within that popup menu give me the option to convert the source file to CMYK. That way I don't have to open everything individually in Photoshop, convert the files then save them.
Otherwise, if this could be done through "Package..." that could work as well. Upon first exporting the Export dialog box warns about all RGB files that are linked. Perhaps there could be a checkbox somewhere to either convert the source files to CMYK, or just collect the linked files then convert them to CMYK thus leaving the source files as RGB?
Ask the press you're sending the files to if they require the images to be converted by you or if there RIP software will handle that automatically and convert to their specific color profile.
Also, if you try just converting to CMYK from image files if the image files have any adjustment masks applied to them for color it will remove them and you lose the adjustments that were made.
RIck Burress commented
This can be done already by exporting as a PDF with Convert Options: Convert to Destination.
AND, there are SEVERAL GREAT REASON NEVER to convert to CMYK—even in a CMYK Print flow.
David B commented
InDesign can convert all RGB images to CMYK when you export a PDF. That is the best time to do it. For more information, see: https://indesignsecrets.com/import-rgb-images-indesign-convert-cmyk-export.php