How did you go with this @zylantex ?
I'm thinking of doing similar but it appears no post/threads can be brought over?
So all I can bring over is the members? What about threads etc. is that a job that requires professional help outside of the importer, or can't be done at all.
The test I did worked very well with one or two minor blips.
You must manually copy via FTP your custom avatars and attachments to the new XF directory structure before you start the import process. I can't remember what directories they have to go in, possibly /images/misc and images/avatars, but you get a warning at the appropriate stage of the import which identifies the directories. A quick directory rename via FTP and the job's done. Some attachments failed, possibly because I didn't get the size parameters correct. I had bent the rules a little on the phpbb forum to allow some particularly interesting photos.
The other issue was videos. The BB code to display video in phpBB does not match exactly with that in XF so it is probably best to run a SQL command to strip the video BB code from your posts before import.
Other than that every post was correctly imported. I was very impressed.