Hi, I have an existing wired alarm system, but would like to add a
wireless door contact in a shed about 20' away from the building.

So I need a wireless door contact in the shed that transmits to a
receiver that is wired into the existing alarm indoors.

Anyone know of such a device?

Thanks!

PS. The reason I don't want to add a wired sensor as that would
entail digging up lots of concrete with drains in/under, or a low
overhead cable