Can Apple MacBook Air be added to Family Sharing for iCloud storage?

Individual devices such as a MacBook Air are not added. Family sharing of iCloud storage plans shares the storage with family members' Apple IDs and applies to all of their devices. See sharing instructions and information in Share iCloud+ with your family - Apple Support

As mentioned above, iCloud is per Apple ID, and not per Mac or device.

As you’re probably already logged into the same Apple ID you’re using with iCloud elsewhere, here’s how to set up iCloud on macOS: Set up iCloud Drive - Apple Support

Unlike what is available for iPhone and iPad, iCloud does not offer a way to back up a Mac.

Hi,

Yes you can. Following article could help your question.

Use iCloud+ with your family - Apple Support