Currently, CA does not provide customers with the ability to sign custom assertions.
To have them signed by CA please follows these steps:
1) Open a case with API Gateway Support Team through the CA Support Portal (https://support.ca.com) requesting a custom assertion to be signed
2) Attach the Modules file(s) to the case.
The team will sign the assertion(s) and provide the resulting file(s) back to you via the case.