I'm afraid while theoretically possible (albeit not in the Magix program) this is practically not feasible. Footage that you want to use for chroma keying effects need to be shot in a studio in front of the appropriately colored background. While manually isolating an object in a still photo works fairly well, trying to apply this technique in the world of motion video would be an extreme challenge and not only require a lot of time and patience, but also a highly specialized software that supports this operation.