Check out this mod made by AlienSlof that adds a new robe and hood based on Xardas' robe.
This mod from AlienSlof adds a new robe and hood based on Xardas' robe from the Gothic series of games to the Colovian Traders in Skingrad. The outfit comes in clothing, light and heavy armour at affordable prices. Look for 'Robe of the Dark Arts' in his stock list.
*Note: I've set this robe up so it can be worn with greaves or trousers under it - it takes only the upperbody slot. There may be some clipping depending on what's being worn under it; obviously tighter-fitting items will clip less.
Robe is male only.