OK found :

Copy the .bz2 file via WinSCP (must be installed) to Windows 7, unzip with WinZIP to a normal tar file, then copy back the tar file to the N900 and then ...

$ tar -cvf rootfs-f12.tar ... bla bla

I hope it helps ...

Alternate procedure :

Unzip the tar file on Windows with WinZIP and MOVE files to the N900 Linux partition via WinSCP