Currently, the addition of connecting and stiffening elements to thermoplastic composites via injection molding is state of the art in the automotive sector. However, the production of injection molded components can only be economically implemented for large quantities due to the high mold costs, which also reduces the customizability of these elements. For industries that demand higher customization and require a smaller amount of produced parts, such as the aerospace industry, the use of additive manufacturing is an excellent alternative to provide these functionalities. The features can be printed directly on the main part using, for example, Fused Filament Fabrication (FFF). This research aims to investigate the functionalization of injection-molded polyethersulfone parts by direct printing on the surface with polysulfone using FFF and understand the nature of the interfacial bond. Infrared microscopy was used to analyze the thickness of the interdiffusion zone and pull-off tests to determine the adhesion strength. With these results, we discuss the influence of the printing parameters (printing speed and nozzle temperature) on the thickness of the interdiffusion zone and its effect on the adhesion strength between the two components. It was found that at fast printing speed, the nozzle temperature has a negligible effect on the adhesion strength. In contrast, nozzle temperature has a strong effect on the pull-off strength at a slow printing speed. The temperature determines the melt viscosity, which directly influences its ability to wet the surface. The melt also transfers heat to the surface, increasing the polymer molecules' mobility. With decreasing the printing speed, the amount of heat transferred to the surface increases resulting in higher interdiffusion width and stronger interface.