This situation is no different, you just have to validate against the main schema.
But, in order to use XMLType's schemaValidate method, you have to register the schema in the database (via DBMS_XMLSCHEMA API).
If the main schema contains imports then those referenced schemas have to be registered too.
To sum up : register all 3 schemas in their order of dependency, then you'll be able to validate your XML against the main XSD.
schemaURL => <schema url goes here> e.g. 'xxxx.xsd'
, schemaDoc => <schema content goes here>
, local => true
, genTypes => false
, genTables => false
, enableHierarchy => dbms_xmlschema.ENABLE_HIERARCHY_NONE