How to rename a batch of files like "01 Hello" to "01 - Hello"?

General Discussion
2

Use the function Convert>Tag-Filename
Format string: $num(%track%,2) - %title%
(provided the tag fields are filled)
Otherwise you should fill the tag fields first.
E.g.
function Convert>Filename-Tag
Pattern: %track% %title%

There was a similar problem some time back: