Hi Christian,
In all cases that I have been involved in, it is from MODFLOW to MSHE. Even in this case, it is relatively infrequent because most people prefer to rebuild the models from the original data.
The ease of conversion from MSHE to MODFLOW will really depend on if you are using a GUI, or not - and which GUI.
1. Properties: if your GUI supports shp file imports, then you can export all of your property maps to a shp file by right clicking on the map in Pre-processed tab.
2. Boundaries: boundary conditions are more difficult because these are model specific. For example, there is way to convert a M11 model to the MODFLOW stream package.
3. Time series: MODFLOW uses stress periods and the GUIs handle time series in different ways. So, it is probably better to simply re-import rainfall, pumping rates, etc directly into the new model.
I realize this is not a great help.
Regards,
douglas