It should be possible with Regular Expressions, that Florian said he would add to its "internal wishlist".
I don't know any way to do it for the time being.
I've implemented regular expressions with the current development build of Mp3tag.
Just choose Converter, Actions and create a new action group called TagsLikeThis and a Replace with regular expression action with Regular Expression: 'u+' Replace matches by: ' $0' (note the space character)
and a second Replace with regular expression action with Regular Expression: '^s' Replace matches by: (keep this field empty)