There is no decision node for WESB itself, but there is a supportpac for WPS. So that would be one approach.
From WESB you're probably going to connect to a standalone Rule Execution Server over a web service call. So just export the WSDL for the rule set, either from a Hosted Transparent Decision Server, a Managed Transparent Decision Server, or another Web Service. I normally export the WSDL to a file system before importing it into the WESB or Messge Broker tooling, but that's just my preference.