I want to define a complex data type in the yawl editor that includes being able to upload varying amount of documents.
But i can't use YDocumentType in the Data definition Editor because it is a Yawl internal data type and not part of this Schema: http://www.w3.org/2001/XMLSchema .
Is there a possibility to still use YDocumentType somehow?

This is the answer from Michael Adams: YDocumentType is a YAWL internal type specifically designed and used to support document passing and storage within a YAWL workflow instance. As such, it cannot be redeclared, extended, restricted or used as a type for a member element of a user-defined complex type.