Solid-state fermentation is a potential technology for developing lignocellulosic biomass-based biorefineries. This work dealt with solid-state fermentation for carboxylates production from corn stover, as building blocks for a lignocellulosic feedstock-based biorefinery. The effect of extrusion pretreatment, together with the action of a microbial consortia and hydrolytic enzymes as biotic triggers, was investigated on corn stover conversion, microbial metabolic pathways, and populations. The extrusion caused changes in the physical and morphological characteristics, without altering the biochemical composition of the corn stover. Extrusion also led to remarkable differences in the composition of the indigenous microbial population of the substrate. Consequently, it affected the structure of community developed after fermentation and the substrate conversion yield, which increased by 118% (from 23 ± 4 gCOD/kgVSi obtained with raw substrate to 51 ± 1 gCOD/kgVSi with extruded corn stover) with regard to self-fermentation experiments. The use of activated sludge as inoculum further increased the total substrate conversion into carboxylates, up to 60 ± 2 gCOD/kgVSi, and shaped the microbial communities (mainly composed of bacteria from the Clostridia and Bacteroidia classes) with subsequent homogenization of the fermentation pathways. The addition of hydrolytic enzymes into the reactors further increased the corn stover conversion, leading to a maximum yield of 142 ± 1 gCOD/kgVSi. Thus, extrusion pretreatment combined with the use of an inoculum and enzyme addition increased by 506% corn stover conversion into carboxylates. Beside biomass pretreatment, the results of this study indicated that biotic factor greatly impacted solid-state fermentation by shaping the microbial communities and related metabolic pathways.