First off 11.04 has expired. Not a good place to be if you need to make
changes as the repositories are not available. I would suggest that you
install a newer version. I do not think that you can even upgrade at this
point. The second point is that the Classic desktop is no longer supported.
It was meant to be a transitional desktop. The choices now are Unity, GNOME
Shell and Cinnamon or Mate via PPA. T

It is pointless to tell you how to do what you want because you will likely
run into problems and get yourself into a problem that you cannot fix. I
suggest that you backup and go for fresh installation, probably of the LTS
which has five years of support, 12.04.