There are several ways of switching between finger picking and strumming, but I believe that Joan used the "Carter scratch" for strumming, which makes it easy to switch to finger style playing.
The Carter scratch is played by using the thumb for the 1 & 3 beats and the back of the fingernails, followed by an up stroke brush with the fingers for the 2 & 4 beats. For waltz time, use the thumb on 1 and the fingers on 2&3.
Hope that helps Guest Megan R.