I was in the same place a few months ago, but in the end I decided that importing SMFs private messages and polls just wasn’t worth the effort. Polls weren’t used in my forum and private messages in SMF are just too weird: you can collect a string of messages into a conversation as would be required for Discourse’s idea of PMs, but SMF stores recipients for each individual (private) post.
A generic solution would either have to split an SMF2 PM thread into several Discourse PMs, one for each unique, non-contained set of participants, possibly duplicating messages to achieve this, or make all private messages in a conversation visible to all participants which might break confidentiality… 