I've looked around and haven't really seen any definitive information out there. Last I heard was that you can only port to GV from the "big four" (Att, Tmobile, Sprint & VZW) and nobody else, even the subsidiaries.
Has anyone been able to recently port to GV from Page Plus though? I'm considering doing this so I'm not locked into PP in the long-term. At first, I was avoiding the $20 fee but I figure it's probably worth it considering the portability and freedom I'd have moving forward.