The problem with Kimimaru's implementation is that you cannot use it in real time, this action will stall the whole game, so doing it every frame, for example, is not feasable (for consistent high framerates). This is because GetData/SetData transfer the memory from GPU to RAM in order to execute these commands on the CPU. But because GPU/CPU render cycles are shifted a bit this means stalling the pipeline.
In general GetData/SetData should be avoided at all costs, unless you are willing to take the hit or deem it absolutely necessary.
So an actual option is to write to your rendertarget on the GPU.
You can do that with monogame's spritebatch, for example, but you could also use a simple texture read write shader.
Spritebatch.draw ( old RT)