The problem with this, for me, is that once I got into the store, both baby and I would get HOT and uncomfortable, so if I was wearing a coat/layers *under* the carrier (and if the baby had layers on whiel riding in the carrier), I would have to take the baby out and take layers off, and then put baby back in the carrier once in the store, and honestly that wrecked the whole idea of ease and convenience for me. I was able to get a hat off of baby while still on my back, and the coat that was around both of us would come off and we were in our regular clothes. And then neither of us was sweating in the store. But we are a sweaty family.
they do sell fleece covers for carriers - that would keep baby warm while you run into the store but would be easily removed and popped in your bag once you get in.
this is why i like the poncho though. it pops on over us very easily. so if it's just a quick run into a store and you don't want under layers, throwing the poncho over you and baby with regular clothes on would keep you both covered while you run in, and is easy to pop off as soon as you get in. a babywearing coat would work too.
as for converting existing coats, i agree with the idea of a panel in the front to extend the width. that way, too, it is just a normal coat when you don't have baby on you. and babywearing coats are very cool if you can afford one. i found that, for the weather where i live, and my budget, the poncho was sufficient. but in a colder climate, i think the investment in a kindercoat or similar would be well worth it.