Yes, but only for other Apple Airport base stations.
Added problem with extenders is a fairly substantial throughput hit. Media servers won't be happy. Consider wired Ethernet where possible and Ethernet over power vs WiFi.
If you must go the WiFi route and don't want a throughput penalty, look into setting up the routers as a roaming network. You will have to wire all the routers together via Cat5e and not every router supports roaming (the Apple AEBS does).
Is it possible to create a roaming network using Ethernet over power line adaptor to get Cat5 to the 2nd APE?
Can't see why not. Ethernet over power is just going to look like wired Ethernet to a device. It will have less speed (~200Mbs depends on adapter) and more latency but if you can't run a wire it's the next best thing. Added bonus (or not) depending on your point of view is it should cover the whole house with ease, and might work on a few of your neighbors homes too. Don't worry the adapters are encrypted.