I don't fully understand the question. Why would you have to save frames one by one, for example? You can save LightMix output by a) Ensuring you have the Corona Multipass Settings to have the LightMix and LightSelect passes enabled and b) Ensuring Save is checked under Multi-Pass image in the Render Settings / Save section (I'd suggest not using PSD as the output though, and just have everything saved to separate PNGs). Then you will get the Beauty pass (no LightMix), the LightMix pass, and all the LightSelects, for each frame (you can delete the ones you don't need after rendering is complete).