FB Plus "Routine" Creation Feature

I'm brand new (day 2) to the Plus. I joined for the Plus challenges; currently doing Booty, Flex and Fundamentals and would like to work in some independent Core workouts as well. The prompt to create a "routine" popped up when I was adding the Core workout to my calendar, which was great as I hadn't thought to create/save it that way. The issue I'm having is that, when following the prompts to create a "routine", the challenges aren't able to be added. Is this really the case? Can I create/save a "Routine" with a mix of challenges and independent workouts or only with independent workouts content?

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