XF 1.5 About the error message 'User names must be unique....'

I'm migrating user accounts manually from IPS to xenForo 1.5.8.

but I cannot register some users to xenForo because of the error message 'User names must be unique. The specified user name is already in use.' in /data/xenforo/library/XenForo/DataWriter.php:1359

I guess the error is occurred by the user names like Jøsh, Josh or Nerø, Nero.

I would like to allow all the user names. How can I solve this problem? Is it possible to solve the error without modifying any source codes?