I think there was a way to customize the existing characters, like add a turban or backpack. Ironically the content about that is so old I can't find a single link about it anymore. I think Autodesk recently culled posts over 10 years old from their search too so it's gone gone. I tried some search engines too (like duckduckgo, youtube, and google) but they are so bad these days I think they are lost down the memory hole forever. (Can we say the information age is now over? I think so.)
I'm not sure about fully custom characters or animations. If memory serves that's not as cut and dry with Populate. Many folks are using either Anima or TyFlow for crowds these days, pretty cool. Certainly more customizable than Populate but requires some VFX knowledge. (Not a lot for stills.)
If it's just for stills and you want a free solution I would consider using particle flow on the meshes to easily spawn them around the scene. Then you have full control and it's already free in 3dsmax. You'd need to detach meshes in groups to render because you don't want a whole crowd as an object. Certainly faster than posing and placing them all manually. If you have forest pack you could use that as well rather easily. Then you get the bonus of all the speed and rendering optimizations it has for scattered objects.
At my small studio we've been experimenting with a local server of stable diffusion to add crowds to stills using img2img. Like anything, it isn't perfect but the cost savings are significant and it matches lighting pretty well. We don't do many pre-rendered things these days so I don't have it down well yet. Big learning curve on that though, especially the python environment configuration and setup.
Best Regards,
