Most of the data sources that Alloy is integrated with can interchangeably use SSN and ITIN. If the data source can accept an SSN, it can likely accept and validate ITIN as a replacement for SSN.
Additionally, some data sources return a flag (e.g., reason codes) saying that the supplied "document_ssn" data is an ITIN (vs. SSN).
Simply pass the ITIN in with the SSN attribute in the supplied data:
"document_ssn": "ITIN HERE"