Rjb-1.2.9 contains new method Rjb::add_jar.
At anytime Rjb can accept to add new jar.
ex)
require 'rjb'
Foo = Rjb::import('com.example.Foo')
...
Rjb::add_jar('/usr/java/lib/junit-4.3.jar') #
TestCase = Rjb::import('junit.framework.TestCase')

Currently add_jar methods only accept single String argument that contains fullpath name of the jar.