Well, you’re out of luck! Cast Iron can create a hash with MD5, SHA-1 and SHA-256, but this is just a hash. If you want to create a hash with a secret key that you’ve been given, ie sign the message, all Websphere Cast Iron can do is create this signature with a SHA-1 algorithm.
I’ve created an RFE (Request for Enhancement) dated in May 2013! but no sign of this even being considered yet.
If you find yourself looking to integrate to cloud apps that require signing a message with anything other than SHA-1, Cast Iron won’t be able to do it at this point in time.