Converting CMYK to RGB

InDesign lets you mix objects with different color spaces and profiles on the same page. So if you are wanting to sample colors from placed CMYK objects and use it for type fills, you can use the Eyedropper tool.

For images the sampled color's mode will match the mode of the image, for vector art it will be a color managed RGB color matching the appearance of the vector's CMYK color. Any sampled color or swatch can be converted into a different color space, and the conversion will be color managed. So in the case of CMYK images you can sample and convert the CMYK color into your document's assigned RGB space, and use it for type color fills, via either the Color Picker or the Color panel.

If you want to export the entire document to a PDF in a single RGB color space (i.e., sRGB), you can do that by setting the the Destination in the Output tab to the desired RGB profile. Interactive PDF exports convert all color into profiled sRGB.

from indesign you can export an interactive .pdf (RBG) or print .pdf  (CYMK).

you don't gain colors when going from CYMK to RBG. When you covert from RBG to CYMK colors are lost. you however can't put colors back/add by converting.

please explain further on what you are looking to do

RGB typesetting? For what purpose?

And, I hate to tell you this but once it's CMYK, you've effectively thrown away too much data to convert it back to RGB without it being quite lossy. The color gamuts are just too different.

If you can back up and tell us exactly why you're trying to do this, maybe we can help but also tell us your InDesign version and operating system.

Why not just led InDesign concert as needed? What specific reason do you see for converting the assets first?