I had to rapidly find a solution, but had no time to study how to redesign the lvm partition table.
Since I had and unused disk I installed system-config-lvm and created a new partition.
Than I had to find a way to move the /usr contents, preserving links and permissions.
This was the solution:
# mkdir /mnt/usr # mount /dev/newlvm/partition /mnt/usr # (cd /; tar cf - usr) | (cd /mnt; tar xf - )
The last line is a poem: it use a pipe between two tar commands, executed on different folders.
The first tar is executed in root and send to the standard output a backup of /usr.
The second tar is executed in /mnt and extract the backup it recieve from the standard output.
There is a great power in such a command.