I hope I understand you correctly: Yes, you can exchange the models in almost every custom scene with your own models, simply by selecting the person atom and then choose/load one of your appearance presets. This method keeps the scene most likely intact. To create a appearance preset of the original character model, just open the starting (default) scene, select the person atom and save a appearance preset. There are other ways, but this is the most reliable afaik. There are other forms of presets as well, so you can choose what you want to excange.