Adding elements later

The first consideration is lighting. The lighting on the new object has to match the direction and angle of incidence of the lighting in the scene. The more lights in the scene, the harder it gets. If you’ve decided to render something separately, then pay attention to the original scene

and try to set the lights up to match. A secondary issue is the colour of light. A figure rendered in white, studio-type lighting will look out of place in a sunset where the colour temperature is much lower.