Sponge is a coremod for Forge, and Forge has supported BungeeCord since 1.6 or 1.7. I’ll try to find a link to back that up, I know I’ve seen someone say it.
I just looked at the source for BungeePortals on GitHub and can say that it will need to be ported to Sponge.
Edit: Here’s a link - How will Sponge work with Bungeecord? - #12 by blood