Can I extend my guest network with an Airport Time Capsule? My Apple Airport Extreme (3rd generation) is configured as a router and creates a main network (dual band: 2.4 and 5 GHz)) and a guest network. My Time Capsule (1st generation), configured as a bridge, connected by ethernet to my Airport Extreme and using the same wifi network name and password (roaming) will extend (on 1 band only: 2.4 GHz) the distribution of my main network. 

I know that, if I configure it as a router, I cannot use it to create a guest network. Here it is just a bridge. 

Will it distribute the Aiport Extreme's guest network ?
Yes. If the extending Time Capsule is in "Bridge Mode" (in Airport Utility settings), it should extend both your main and guest wireless networks. This is how I have my network configured.