Currently only the the elements defined in the AssertionType (check NhincCommon.xsd schema) are supported. You can modify the source to add custom assertion attributes, below are the steps:
1) Change the AssertionType in NhincCommon.xsd schema (Add a new element with name/value or any type based on your requirement)
2) Generate Common Types and Connect WebServices jars
3) Use the Common Types and Connect WebServices jars in CONNECT
4) Add the respective getters in gov/hhs/fha/nhinc/callback/openSAML/CallbackMapProperties.java & CallbackProperties.java
5) Extract the value from the entity and set it on the request context --> gov/hhs/fha/nhinc/saml/extraction/SamlTokenCreator.java
6) Create the new Attribute (Attribute statement) that you need to pass to the other gateway --> gov/hhs/fha/nhinc/callback/openSAML/HOKSAMLAssertionBuilder.java & gov/hhs/fha/nhinc/callback/openSAML/OpenSAML2ComponentBuilder.java
Let us know how it goes, please contribute the code to the community if possible.