Support for web service tokens is being considered as a product enhancement for a future version of Identity Manager.
If using the currently supported authentications methods (anonymous and basic username/password) is not possible, a workaround is to utilize the CA API Gateway. Add the necessary X509 cert into the SOAP body in the Gateway policy. The Gateway will communicate to the web service, and the Policy Xpress data element connects to the Gateway, secured with username/password.
More information on CA API Gateway can be found here: