Yep, you're describing the most common magister robe model.
The easiest method to do what you're asking:
In your mod, go to the Divinity Origin armor stats. Copy the entire FTJ_Alexandar_Armour stat to your mod's armor stats. Change the ItemGroup to FTJ_HarbourMagister_MageArmour. Save. (-:
Note without additional changes, the name of the armour will appear as Magister's Mantle, but the stats will all be of Alexandar's.