Some time it is easier to ask directly instead of doing weird things
:-( Sorry for not being clearer.
The point is that by using macro.vm I can easily add new macros to our
sites without needing to modify macros.vm. This way, our customizations
are "completely" independent of the base skin and updates are pretty
straightforward.
Now that following your advise I have learned how to control panels'
appearance by using CSS, it will be great to have a tool to include new
CSS code in the "rendering process" without modifying the default css
files. This way, as with the macros, I have not to be concerned about
overwriting old default css while updating.
The only tool I founded in xwiki.org that I guess could do something
like this is MergeCSS. But I don't understand well how it could be used.
Hope this is a bit clearer now. Thanks for your patience!