You can just drop the xml-rpc.jar in your deploy directory, the classes will be added to the server from there. However you need to make sure the deployment order is correct, so that the classes are deployed before your MDB attempts to instantiate them.
Or drop the jar into your lib directory. It's always deployed before your MDB. The lib dir is not hot-deployable though.
Or package them both into an EAR, just keep the JAR at the root of the EAR directory structure.