Without any details at all about what is in your SP site, that is a pretty big question. I am not sure there is any definitive "tool" that does it automatically so you'll likely be faced with migrating content from one to the other. The bigger thing you'll need to do is find a way of changing usage behavior from living in SharePoint (which is sad) to living in Confluence (which is orders of magnitude better).
Just about every SharePoint site I've ever seen, in the final analysis, amounts to being a clumsy file server for static and discrete documents that nobody can find. Confluence, on the other hand, makes it extremely easy to create actual content vs. static attached documents. As such, I might suggest at least starting with making a framework in Confluence that sorta looks like the organization of SharePoint so people can at least have a learned tree structure to live in. Then, rather than moving static documents from one to the other, actually import the documents into Confluence as content but don't put them in as attachments.
You'll be surprised how fast people get used to not having to:
Getting the information you want RIGHT NOW in Confluence as a page is so refreshing. The search function is stellar to find stuff. Linking referred information between each other is desperately simple (but please don't use hard URL links; use the Confluence linking so that if a page is renamed or moved the link doesn't break)
So... you have a long way to go, I am afraid BUT it will be well worth it. I might suggest you spend some time getting familiar with what Confluence will do first so you have a better idea of how to turn what used to be a painful to use file server into an honest collaboration portal. Look also at the many, many, many plugins available as you can quickly and easily HUGELY extend the capability.
Years back when I got people off the (fairly complex) SharePoint site to Confluence it was a bit slow for usage and collaboration to take hold... but once people got used to the speed and ease of creating content, working together on said content and the like, contributions AND collaboration went through the ROOF. The corporate standard ("you MUST use SharePoint") was still attempted to be pushed down my throat and we took heat for a while for not using SharePoint. In recent times, however, they started to really see what it does AND now that is becoming a preferred portal and is "officially" acceptable now.
>Just about every SharePoint site I've ever seen, in the final analysis, amounts to being a clumsy file server for static and discrete documents that nobody can find
That. That's exactly it. That's why I hear people call it "Does not Share...point".
Every time I've looked at a Sharepoint installation with a view to moving it to Confluence, we've discovered tens, if not hundreds, of documents all covering similar subjects in slightly different ways. It seems like everyone attempting to use it basically can't find what they need, so they write their own version. Don't get me wrong, that happens in Confluence too, but at least, with a bit of training and thought, you can minimise it and encourage people to work together (impossible in SP)
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