Thank you Soma, It worked like a charm. Now i am trying to understand how )
I want to add these classes just because i am trying to follow some css recomendations like SMACSS, OOCSS and BOM. They suggest not making those nested css rules, at least not making them too deep. If one uses ""ul ul ul" type of rules, he has to redefine each next level. So if not putting special classes in ul tags i would have went with "ul > li > ul" rules instead.
Anyway, level placeholder could be used frequently. Of course i really suggest adding it to the core as i need it, but it is because i need it of course )
By the way, i couldn't find any documentation on those template strings and placeholders, which could be used in them. Please lead me to the light if you will.