Hi and welcome to the forums.
The item you are perhaps looking for is called a wireless repeater and there should be several choices on the market if you search under "wireless repeater".
In actual fact any wireless router or wireless access point should in theory be able to act as a wireless repeater, but I have seen reports of incompatibilities between various makes of equipment.
For example if you have a Netgear router then try a Netgear WG602. - If you have a speedtouch then stick with speedtouch etc etc
You may want to have a read of this
http://www.tomsnetworking.com/2004/04/14/how_to_wds_bridge/Bearing in mind that you say your existing wireless connection only just stretched to the door, then the signal at that point to be repeated is going to be fairly weak anyhow.
So since the outhouse has power.. so you may be best looking at wall plugged ethernet..
Theres a choice of products from
here.
I notice that theres even a NetGear 54 Mbps Wall-Plugged Wireless Range Extender Kit
http://www.dabs.com/ProductView.aspx?Quicklinx=3Z8HHope this info helps