In the past, I have taken raw images, and created a jpeg in the camera. I have not done it allot, so I am not fluent with it, but if your need for jpeg is rare, you might consider this.
Update: I got my camera in hand, and found that an approach is to make a copy of the raw, and convert that copied image to a jpeg, then save it.
That does not address having a function key perform this task, but if the need for a jpeg copy were infrequent, it may suffice.