Howdy,
Just about everything that is related to user content such as blogs and blog comments map back to a users ID. And until till you get the users in Dolphin, you don't know their new user ID.
One possibility is as part of the user import process, you use a field that is not part of the join forum, profile edit / view that contains the Ning ID. Then you can translate Ning ID to Dolphin ID.
So when it comes time to import blogs you have the required IDs for users. But before a blog post is imported, the blog in dolphin has to be made. This is not too hard if you want everyone to have a blog, but will require more work to only make blogs for people that had them. Then posts can be assigned to that blog. Same thing with comments.
Through IDs for people and blogs you get a mapping that is like this: Comment > Blog Post > Blog > User
I have not seen a Ning blog export, so I don't know what we will have to do to translate the data into something Dolphin understands. Without doing some testing I'm not sure if it is the comment parent or object id that links back to the post. Maybe someone knows for sure and can share.
Light man a fire keep him warm for a night, light him ON fire & he will be warm the rest of his life