You need to create two files. Mobs.csv and MobItems.csv. They have to contain the same like PS_GameDefs.dbo.Mobs and dbo.MobItems. Open your Monster.SData in shStudio, click on File -> Import.. and choose the csv files you created. Save and you should have the MonsterEx.Sdata. You are able to edit your Mobs in any way.
Cheers