From my understanding, there are a few different ways to go about it. One method is to use a generic extraction program (such as Ninja Ripper) to capture models from a game as it's played; I don't really have experience with this method, though, so I can't comment on it much. The way I use is to look for more specific extraction tools (e.g. the Unity Assets Bundle Extractor for Unity games) that convert specific file formats, which generally dump the models as .dae or .obj files (it's worth noting that one would also need to find the image textures that go with the model), which can then be imported into a 3D program like Blender.
If such a program doesn't already exist and one is *really* good, I think one can also parse through game files using a hex editor or develop custom scripts to extract a model. I know basically nothing about this process and only include it here for the sake of completion, but some (such as Random Talking Bush) have written and posted a bunch of scripts on this forum that import various files directly into 3DS Max, skipping over the need to export an OBJ or DAE.
Anyway, once the files are in the 3D program, the next few steps would be to load the textures onto the model, clean it up if needed and ideally add bones (if it doesn't already have them) to make it poseable. After that, you can export it (ideally as a .dae file if it has bones and as a .obj file if it doesn't have them) and submit for approval. Hope that helps!
If such a program doesn't already exist and one is *really* good, I think one can also parse through game files using a hex editor or develop custom scripts to extract a model. I know basically nothing about this process and only include it here for the sake of completion, but some (such as Random Talking Bush) have written and posted a bunch of scripts on this forum that import various files directly into 3DS Max, skipping over the need to export an OBJ or DAE.
Anyway, once the files are in the 3D program, the next few steps would be to load the textures onto the model, clean it up if needed and ideally add bones (if it doesn't already have them) to make it poseable. After that, you can export it (ideally as a .dae file if it has bones and as a .obj file if it doesn't have them) and submit for approval. Hope that helps!