I'm sorry, but wpForo doesn't have a topic description field, so it'll not be imported.
This tool is designed for default forum components migration like Forums, Topics, Posts, Attachments, and Users. This will not migrate data like PMs, Polls, etc...
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.