Thats a tricky one, as the guest network wireless is isolated, however there is a solution, and that is to use a wireless client to connect to the guest wireless network, and that will give an Ethernet output which can be passed to the annex.
At the annex end, if wireless is needed, then you can plug a wireless access point in there.
For a wireless client, use a TP Link WA701ND, that would be located near your main router, and would act as a bridge.
You set it up so it connects to the guest wireless network.
If you need more help, then please ask, I could make up a diagram.
You could try the WA701ND as a wireless repeater, but there may not be a good position to locate it, and performance would be reduced.
How many connections are you expecting in the annexe?
I have details on the WA701ND if needed.
Edit:- I have just realised that I am doing the same thing. My guest network is used by next door, but there is a computer they use which does not have a wireless card. So I fitted a Wa701ND in there, which connects to my guest network, and plugs into the LAN port of their computer. It was the best option anyway, as the computer is in the part of the house that does not get a good signal, so there is an Ethernet cable from the WA701ND, to the computer.
Its been in there so long, I almost forgot about it
Thanks Keith, sorry for the delayed response.
Think this will be a winter project.
No problem, its actually fairly straightforward, you just have to allocate separate static IP addresses for the wireless client, and access point at the other end. If you set them up beforehand, using a desktop or laptop computer which has a LAN port, then its just a case of connecting them up to each end of the Ethernet cable.