I use the extension for conversion, so I'm not sure if this is the right tool, because:
hg convert svn://svn blah
gives:
svn://blah does not look like a Subversion repository
How to do this if my repo does not have anonymous access?
You need to use the http URL in your subversion repository, or better yet, bring all the local svnsync value and use the URL file . Here are some more details:
http
file
https://www.mercurial-scm.org/wiki/ConvertExtension#More_about_Subversion_URL_and_Paths_Handling