A superparamagnetic (SM) to ferromagnetic (FM) phase transition was investigated in Co-doped (∼6) electroless plated Ni arrays. The introduction of Co altered the microstructure of the Ni arrays from nanocrystalline to polycrystalline, resulting in a SM→FM transition. This Co-induced magnetic phase transition is similar to that observed after heat treatment of undoped samples [C. M. Liu, Y. C. Tseng, C. Chen, M. C. Hsu, T. Y. Chao, and Y. T. Cheng, Nanotechnology 20, 415703 (2009); C. C. Huang, C. C. Lo, Y. C. Tseng, C. M. Liu, and C. Chen, J. Appl. Phys. 109, 113905 (2011)]. The role of Co dopant was identified electronically using x-ray magnetic spectroscopy, revealing that the transition modified the Ni host's electronic structure and enhanced its moment by effectively spin-polarizing the Ni 3d conduction band. This was distinctly different than in the heat treatment case, which underwent an electronically independent phase transition. The element-specific magnetic hysteresis of Co and Ni was also probed, which showed that the two elements were magnetically coupled.