Volumetric additive manufacturing (VAM) is an emerging vat photopolymerization technology that boasts improved fabrication speeds, material isotropies, and surface finishes. However, as VAM is only compatible with highly transparent polymer resins, it is extremely challenging to fabricate composites, since it involves the use of resins that contain light-scattering fillers. In this work, we circumvent the transparency criterion by synthesizing the fillers in situ post-fabrication, thus enabling the fabrication of composites using VAM. To demonstrate the versatility and utility of our approach, we converted Xolography-printed hydrogels into magnetic and conductive hydrogel composites with filler weight fractions of up to 65 wt %. We further show that filler growth can be spatially controlled, which enables the fabrication of multimaterial structures. This work presents a facile strategy for the VAM of functional composites and multimaterial structures, which could pave the way toward the fabrication of previously impossible devices and smart materials.