I was recently playing a mod called Half-Life: Residual Point. I saw that they not only had the HD models pack installed, but they had Massns (if that's what they're called), Race-X monsters (pit drones), Security Guard and Soldier Zombies, and other enemies that were originally only in Opposing Force but he placed them in a mod for Half-Life. Also, none of the main concepts of the mod changed the interface to look like Opposing Force, and it was a mod for Half-Life, so that means that the mod author did not use the Opposing Force .DLL for that mod. Then I checked and saw that he was using a .DLL named survivor.dll in his dlls folder of his mod, and he had a texture file named survivor.wad in the root directory of his mod. In his pak0.pak file, he had models for Massns and some Race-X enemies in the models folder. However, I don't think he was using the Opposing Force .DLL for this mod (the .DLL was called survivor.dll, as I said).
Therefore, how did he use these entities that are not native to Half-Life in his mod without using core files from Opposing Force instead? Does anyone know of how he could have done this? Did he use Spirit.dll and rename it or something? Any suggestions/help is much appreciated, and I apologize if this is a somewhat nub request. I want to include those enemies in a new mod I am developing without making it use Opposing Force's core files (because they don't have a public SDK available like Half-Life does).
Thanks!