Is it possible to set a document class as multifile doc by default?

This should be doable with Property Calculator, for instance as part of a workflow. Multi- and single-file documents are defined by a hidden Boolean property "Single File", so you could use a Property Calculator rule such as this to set Single File = No. The document is converted to multi-file when it reaches the "Convert to Multifile" state in the workflow.

Thank you soo much Joonas, this is fab!