It only returns a subset of user data https://github.com/NodeBB/NodeBB/blob/master/src/posts/user.js#L24-L28
If you want to add more you can do so in a plugin, using the hook filter:posts.modifyUserInfo
Is there anyway to add new fields to topics or posts for custom sorting?
There is no way to do this in core. Are you trying to make a plugin to do this?
For adding fields to a post/topic you would use the hooks as so:
Sorting is another matter, you need to also add the fields to a sortedSet in the db, here are two examples:
You mean add tags after the fact? Only a mod or the OP can edit a topic and add them.
Tags are not available for posts but I really, really, wish they were.