Thanks Didier,

this is exactly the missing function I was looking for, but it does not work works under sketchup 8...
I posted too quickly and I did not put the script in the right plugins folder (damn Apple...)

Here is how to find it if you use a mac : http://sketchuptips.blogspot.se/2008/03/how-to-download-and-install-plugins.html

It works very well and save me a lot of work!!