Sponge is under development by what can be called an "Open Minecraft Alliance"....
The project "Pore" found here: http://www.reddit.com/r/spongeproject/comments/2frfaa/pore_a_bridge_for_bukkit_and_sponge/ May be the most...
you download the latest build from dl.bukkit.org and selecting alternate versions. Then they need to go into your JARS folder in your multicraft...
Separate names with a comma.