For make appear a foreground in C4D in native render it's quit simple, just create a material with PNG transparent in diffuse color or use a PSD file with selected layer.
Create a foreground object and assign it to the material in Frontal projection create above. After that all the 3d objects appear behind the foreground object in render.
For example if you need to make appear a plant or tree in foreground in a same picture than image background or HDRi, just create a alpha or mask or layer of it and it will be appear in first plan in render, don't need to use Photoshop. I hope than I'm enough clear, my English it's not very well. ;-)