I've been looking for something just like this. Does it support migration from Confluence? I glanced over the features section and didn't see mention of that, but perhaps I missed it?
Our API [1] has recently matured to now support the different content types so that could be utilised for such a migration job although the API is still growing to cover more actions/models.
Have recently been thinking about possibly offering some form of paid service support service to help the process and help fund the project.
Fortunately, a lot of the handling is open sourced in atlaskit.