We study the magnetization response of a frozen ferrofluid as large-diameter (10-nm) magnetite particles are added to a dilute, random distribution of small-diameter (5-nm) moments. The small dipole moments are free to relax down to low temperatures, while the large dipoles become immobilized when the solvent freezes, acting as a random local field. The glass transition temperature is a sensitive function of the random field concentration, permitting the investigation of the crossover from spin-glass to random-field behavior.