Miraheze provides imports for all wikis. To request an import please create a Phabricator task with a title of 'Import for [your wikis name]' replacing the name with a link to your wiki and attaching an XML dump. If it is a private wiki you can send this by email to staffmiraheze.org . You can read Moving a wiki to Miraheze if you need help creating a dump file.
Imports are done by the System administrators. They are done using the importDump.php and importImages.php maintenance scripts. The duration of the import depends on the size of the wiki.