I don’t think there is currently a way to do what you describe. There are two decent methods I can think of to narrow down posts from specific users:
Go to the “staff,” “moderators” or any group you are interested in and click the Activity tab to see all posts made by the members of that group. Unfortunately, it’s not a solution on the topic (=thread) level, but it does at least isolate those users.
The other option that does work within the context of a topic is to click on the avatar of the user you are interested in and then select the button you see in this screenshot that says “84 posts in topic.” This will only show the posts in the topic from that user.
As a refinement to this excellent suggestion, you can click on the box at the end of the OP
You can click on Summarize this topic to get the most important contributions (not just staff though) and if these are ‘official answers’ they may well appear on this list
Then if you expand it you can see the list of contributors to identify the major respondents, then do as @tshenry suggests and filter by their contributions alone