I can't find in the documentation where it states to run the mentioned ant commands. Here is the chapter on the installer: http://docs.jboss.org/jbpm/v5.2/userguide/ch03.html.
It should be possible to define your own existing location of AS7 by modifying the jboss.home property in the installers build.properties ( https://github.com/droolsjbpm/jbpm/blob/master/jbpm-installer/build.properties).
As far as existing Eclipse installation goes, the same build.properties file contains a eclipse.home property. Have you tried settign it to your existing installation?