Hmm, I think the OP is referring to powerline adapters, NOT "power over ethernet". Perhaps it should be called EoP (ethernet over power)
Yes, you can just add another powerline adapter. You need at least two to get started, but you can always add powerline adapters one at a time to extend the powerline network. Of course, there's usually an upper limit too (might be 8, 10, whatever, depending on manufacturer).