It's called a buggy pod, I have one. It's not as good as I'd hoped, but I use it for trips to town.
The design is really ingenious, but it falls down in its attachment to the pushchair. That is a pig to get right, then can be a bit of a fight to fit each time.
The fixings stay on the pushchair, and can be ignored if you're not using the pod. I keep the buggy pod folded up in the car. When you think you might need it it pushes on to the fixings. I can see that this ought to be really simple, but as one of the fixings on my pushchair had to go on sideways it's awkward to line up. Nevertheless, I've got better at it.
So, I've got pushchair + baby, and folded buggy pod + walking 3 year old. Now we can go shopping and get in all the shops. When shopping is finished I unfold the pod and 3 year old can ride back. She can sleep in it, although it doesn't look very comfy. It's too wide for some shops, so we do it like this.
It wouldn't be good on rough ground as there is too much flexing around the fixings. There used to be a big problem with all the weight being on one side, but that has lessened as the baby has got heavier. Still a significant pull to one side though.
It's useful, but not as fab as I had hoped. We get many admiring looks, mind you