Is it possible to convert an M14 magazine to function in an Armalite-pattern AR10?
Yes. Thats what Armalite originally did during the Ban era.
Cool! Are any companies currently offering this service that you know of?
As I recall, you got a new magazine body and follower and you used the spring and floorplate from an M-14 magazine. It didn’t really make an M-14 mag into an AR-10 mag.
They also didn’t work so well. At least for me. However the Gen2 magazines worked flawlessly.
YMMV,