Laser powder bed fusion (LPBF) is a metal additive manufacturing (AM) process for fabricating high-performance functional parts and tools in various metallic alloys, such as titanium, aluminium and tool steels. The process can produce geometrically complex features such as conformal cooling channels (CCC) in plastic injection mould inserts to improve cooling efficiency. A recent attempt using a hybrid-build LPBF AM technique to fabricate aluminium mould inserts with CCC attained a substantial reduction in processing time, making it an attractive alternative method to the mould-making industry. Also, the successful bonding of aluminium powder with wrought aluminium alloys proved the practicability of this concept. This study further investigates whether a similarly successful outcome could apply to tool steel since tool steel is the preferred material for constructing high-grade high-volume plastic injection moulds. In this investigation, hybrid 18Ni300 powder-wrought 17-4 PH steel parts were additively fabricated using the hybrid-build LPBF technique, followed by various post-build heat treatments. The mechanical and metallurgical properties of the samples’ bonded interface were examined. Microstructure analysis revealed homogenous powder-substrate fusion across the interface region. Results from tensile tests confirmed strong powder-substrate bonding as none of the tensile fractures occurred at the interface. A direct post-build one-hour age-hardening treatment achieved the best combination of hardness, tensile strength, and ductility. The overall result demonstrates that hybrid-built 18Ni300-17-4 PH steel can be a material choice for manufacturing durable and high-performance injection mould inserts for high-volume production.