1 :
Import model inyo ZBrush (Tool/Import button) and save the current geometry (Tool/ Morph Target/ StoreMT button).
2 :
Divide the model to a higher mesh resolution (Tool/Geometry/Divide) and do the sculpting with Zbrush brushes
2 :
At the end of the sculpting adjust the subdivision level of the model to the lowest value (Tools/Geometry/Lower Res button or Subdivision level slider);
-export it and use this base model and the DispMaps in modo, or
-generate the DispMap for the original modo model. In this case you have to set the imported base mesh of the model back (Tool/ Morph Target/ Switch button).
