There are a few different ways of switching back and forth between flatpick and fingerstyle from what I've seen and read. One method is to tuck the pick into the palm of the hand leaving whatever remaining fingers for fingerstyle. I know that this works successfully for players. And players can become very proficient at this. One example is that Brian Setzer does a demonstration on his instructional video.
While that may be ok for him, I never learned that way. What I do is something very simple and effective. Since I primarily use the thumb, forefinger and middle finger for fingerstyle, I simply tuck the pick away between the ring and middle fingers when the need arises. I can go back and forth between the normal pick grip and fingerstyle at will and with ease. I believe you can to. Also, I can ocassionally use my ring finger for picking. Here's how it works: