Thanks for the reply... I think that custom files are not enough for my need; infact HAML is similar to HTML ( and has some similarity with YAML ) and SCSS is really similar to CSS, but there are some differences that make impossible to use a custom file type: for example in HAML you use code folding ( like in python ), but the way you specify arguments is too different from every supported type. Or in SCSS you use the CSS syntax ( with little modifications ), but you can nest selectors ( which is something that you cannot do ).
Also with a new lexer all scintilla based editor can have HAML / SCSS highlighting.
This reasons make me think that a new lexer is the best way to achive my objective...
Regards, Edoardo Tenani