Java 6 has a feature that can help with plugging external JARs into ABCL: If the classpath includes an asterisk, all JARs in the given directory are added. For example, I've created the directory ~/local/abcl for my ABCL installation that includes abcl.jar plus any other JARs I want to use. Then the following command automatically adds everything in that directory to the classpath and starts ABCL:
exec java -cp "$HOME/local/abcl/*" \ org.armedbear.lisp.Main \ "$@"
(The "exec" can probably be omitted)