Somewhat off topic, but I hope I can leave it here for others (like me) who found this thread hoping to export playlists which they had created themselves on Listenbrainz (i.e. not the weekly generated ones)
This site will not only convert a JSPF into an XSPF but you can input your LB username and export your (self-created) playlists as XSPF!!!
Thanks to David Di Blasio
[EDITED BECAUSE I WAS SO EXCITED I FORGOT TO INCLUDE THE LINK]