Suthriel:
Not sure, if i understand your question correct, but the HDRI backdrop requires panoramic/spherical textures, it´s designed for those kind of textures. However, nothing would stop you to just google a normal flat picture of an interior you like, and then add an additional flat plane into the scene, that will act as your real background, and give this flat plane your normal flat picture as (unlit or emissive, maybe masked too) material. Then you would have the HDRI backdrop for lighting your scene, while the flat plane would simply act as a background object with the interior, that you like.
Thanks. This is exactly what I need for some stages of the demonstration that I intend to produce. I will try to find an example like this in this forum. I am really a newbie in Unreal.