How can I convert my forum to XenForo if an importer doesn't exist?
You can convert to a compatible version of IP.Board or vBulletin (appropriate license required), phpBB or myBB (free) and then use one of the built in XenForo importers.
Note that if converting to vBulletin, ImpEx doesn't preserve group permissions, so users will have no permissions when they are imported to vBulletin and that will carry over to XenForo. You should carefully review the permissions of all imported users and user groups, or you can simply delete the imported user groups and any members will default to the Registered user group. Similarly, passwords may also not be preserved.