Fat Transfer or Fat Grafting can be done in the forehead. Typically Fat Transfer is done in the Temple Area because fullness in the Temple or side of the Forehad creates a youthful appearance.
If your concern is deep Forehead lines such as Frown Lines and Worry Lines, Botox or Dysport are more effective solutions. Fillers such as Restylane, Pearlane, Juvederm, Sculptra and others can be used to plump forehead lines, but without Botox or Dysport to quiet the muscles causing these lines, the results of Fillers are usually disappointing.
If you have a contour deformity or wish to have a more prominent forehead then Fat Transfer or Fat Grafting would be a solution, however I would have fillers first to see if you like the result.