Is there anyway I can connect the Extreme to the Express via a Netgear Powerline? If so, will this work with both 2.4 and 5.0 networks.
Yes on both. Note however, when interconnected by Powerline adapters, it would be the same as though they were connected by Ethernet. The type of extended network would not be an "extended" network but a "roaming" one. Both basically do the same thing.