Hi,
this is one of features that we are working on, and hopefully it will be finished in one of 8.x relases.
We will allow user to configure classification to system types, such as dms:sysobject , that will allow resolution of any document type.
Until it is finished, possible workaround would be to define an abstract type and derive all documents from it (this will not work if you already use abstract types, that is why we are extending the function with that regard)
best regards,
Bratislav