This is currently a bit of a weakness in the platform. I wouldn't recommend implementing category expert functionality - it's an old feature that never worked reliably in my experience and Lithium have since invested in the premium gamification offering (badges) instead. But a t present the badge metrics are only community-wide, it would be great to have more granularity and flexibility with badges.
See https://community.lithium.com/t5/Product-Ideas/Badge-for-starting-a-great-discussion/idi-p/201469 and https://community.lithium.com/t5/Product-Ideas/Badge-for-participating-on-a-particular-board/idi-p/197787 for a couple of enhancements I would like to see to allow you to reward specific behaviour on specific boards.
... View more