I guess your client does not have WebSphere MQ Adapter? Is it so?
In my past experience, we had a utility to put bulk messages in to MQ queue via java service. Just refer the below links and you write a java service in webMethods. Also take care of appropriate jars from IBM WebSphere MQ.
http://www.coderanch.com/t/77875/Websphere/Simple-Java-MQ-Connection
And refer IBM MQ API.
#Integration-Server-and-ESB#Flow-and-Java-services#webMethods