A couple of thoughts;
Purely within DEM you can define any number of process layers, so the activity within the sub-process can itself be just a diagram that breaks down to a number of more specific sub-processes.
Otherwise, if you are using IVc or ERP, you could set up Visio or similar as a helper application or component. While this would only be of use in exploding processes that are not Baan enabled, the diagram produced is not subject to the sntax requirements of DEM. Whether that is good or bad.....