file-roller (the default GNOME archive manager) doesn't use unrar-free. It just looks for 'unrar', which is in unrar-nonfree in Ubuntu and Debian.

To add support for unrar-free in file-roller (except for the most recent versions of RAR files, which can only be opened by unrar-nonfree) I did the following:

 sudo ln -s /usr/bin/unrar-free /usr/bin/unrar

Is it posible to do this automatically when installing the unrar-free package?

First of all i want to say thanks, i was trying to make unrar-free to work, but i supposed it was because of the files i tried to "unrar".
RAR decompression code is copyright (c) by Eugene Roshal. UniquE RAR

   File Library is copyright (c) 2000-2002 by Christian Scheurer and
   Johannes Winkelmann. Unrar is copyright (c) 2004 by Ben Asselstine.

I think is them or the maintainers are the ones that can do such thing.
Maintainers: Ben Asselstine <benasselstine@canada.com> Jeroen Dekkers <jeroen@dekkers.cx>

I think that it's not necessary to contact the authors of RAR. After all, we (well, gNewSense developers) just need to rename a file.

Maybe the authors (or Debian/Ubuntu packagers) won't rename the file because it will conflict with the more convenient unrar-nonfree. So, this modification has to be specific to gNewSense.

Another option is to modify file-roller to allow the user choose the programs to open the different file formats. This idea is proposed in http://brainstorm.ubuntu.com/idea/4204/

Another option would just be to modify the existing package to create the symlink (/usr/bin/unrar -> /usr/bin/unrar-free). I personally like the free moniker on the package.