The npc morph code that was present in the SR2 engine was what allowed you to play as the npcs (once we modded in the new cheats, that is.) Originally those npc morphs were intended for SR2 multiplayer to allow you to play as the npcs, but was changed during the last stages of development. There are even prerelease videos of SR2 multi showing this feature before it got cut.
At any rate, this morph code does not exist at all in the SRTT engine. I think it's because multiplayer itself was left out and there was no reason to include it. Now, there has been some minor success with swapping out clothing meshes so you could "wear" certain npcs as clothing, but it's extremely buggy and only works correctly for a couple of them. The bottom line is that your request is simply not possible with our limited toolset.