@Mruk perhaps I'm bring a little too optimistic/naive in hoping that all forum softwares could be exporting to a common format (and co-exist happily 😆), but even that comes with its own caveats, namely in that it'd only be able to support the lowest common denominator, so just posts, topics, and categories.
The Redis (or MongoDB) dump file would be the best resource for the data, though, yes...
The recommended method of sharing sessions between two separate and distinct applications is through OAuth2. We recommend this approach because NodeBB maintains its own user records, so that we can keep track of user-related metrics and other data. Relying on another database would be tricky, prone to breaking, and quite possibly dangerous.
Luckily, it's quite straightforward to get things working with OAuth2!
The first step is getting your application to expose an OAuth2 endpoint. If you're running a Node.js based app, you can use a module called OAuth2orize.