I'm not sure what the plans are for adding support for CMYK in Fresco but if this is a feature you'd like to see in the app, please post it on https://adobefresco.uservoice.com. You can also access User Voice from within Fresco:
* From the Home screen tap the gear icon in the upper right corner of the screen.
* Then from the App Settings menu go to Help > Support > Suggest a feature
When all requests are all in one place the team can prioritize those features that are most important to the largest number of people.