as you mentioned fields can be set to required on file cabinet level only at the moment. This means they are required across all store dialogs.
As announced in our customer feedback forum, we are going to change that with the next DocuWare Version (~ Spring 2019)
Fields can be set as required on dialog level then. So you can specify fields as mandatory for some dialogs, but not for others.
Until then you might take a look at the Validation, which validates the input when storing via an custom web service.
Viele Grüße / With best regards,