Are you asking about the actual conversion of data or getting the “community” on board?
For the actual conversion, we did it manually 
. We didn’t see an existing importer, and for the size of our wiki it didn’t make sense to develop one. I sat down with 3 other employees and we copied and pasted everything over. Only took an hour or so to do, plus another hour or so to go back and fix formatting and links, plus upload attachments.
As for getting the “community” on board, as they are employees and I am in-charge of our server, they really didn’t have a choice 
. The main reason for the switch was that no one liked the discussion functions of MediaWiki, which meant that pages were frequently updated without prior discussion, leading sometimes to confusion, and in a few cases back-and-forth edits (from users who didn’t understand how to view history…). With Discourse, we’ve seen that staff are much more likely to add a reply to a topic than just edit, which leads to good discussion and eventually an edit. The only complaint so far about the switch is completely unrelated to wiki functionality (email stuff).
Honestly, this feature (really the lack of it) is the biggest thing preventing me from saying “Discourse is perfect for this!”.