Transfer family member's iCloud data to family storage

A family member has their own iCloud storage. I have just obtained family storage, which is on my own Apple ID. How can my family member transfer their data from their own iCloud storage to our family storage?

I have seen posts about Apple having no formal transfer process. I also have seen posts about someone buying a Mac and wanting to use their own Apple ID for it instead of the one that was on there previously. My situation is different because my family member will retain their own Apple ID on her Mac and will just accept the invitation to family storage.

I also know that my family member can transfer her iCloud data to her Mac and then upload it to the family storage. I'm just wondering if there's another option.

Thanks very much in advance for any suggestions.