PolConvert is implemented as a CASA task and may be installed
as described in src/INSTALL.  The machinations at this level
are needed to integrate it into a standard DiFX build process.

The installed directory contains the elements needed to use
polconvert from within CASA.  Typically you might add the
following line to your .casa/init.py file:

try:
    from os import environ
    execfile(environ['DIFXROOT'] + '/share/polconvert/mytasks.py')
except:
    print 'unable to load $DIFXROOT/share/polconvert/mytasks.py'

(or similar).  Create .casa/init.py if it does not exist.
You can then run polconvert directly from within CASA following
the sample input commands provided in (src/PP/)README.POLCONVERT
which should be installed in DIFX_ROOT/share/polconvert.