We present the development of an all-fiber side-fused signal-pump-combiner based on an integrated 3C R feedthrough fiber. This specialty fiber uses a 34 μm single-mode core and shows great potential to enable further output power scaling while maintaining high beam quality. The side-fusing technique has the advantage of an uninterrupted signal core and can be used in co- and counter-pumped fiber lasers and amplifiers. The signal-pump-combiner was operated up to an input power of 600W from four pump fibers and coupling efficiencies of 79\% were achieved. The component was additionally investigated by computer tomography imaging, which revealed that the cladding structure of this specialty fiber prevented the required level of glass fusion of the 3C R fiber with the pump fibers. The investigation will help to further increase the pump coupling efficiency of the signal-pump-combiners. This represents the first step of developing all-fiber and high power capable laser systems based on the 3C R fiber.