lionel milwaukee 261

I dont think you can

Press and hold AUX2 which will bring up the lighting keypad, there should be Mars ON and Mars OFF.

Actually you need the new orange chip that has the updated toggle off on it. Once you reload this into the Cab2 the icon will appear. The original orange module did not have that option to turn it off.

Can it be added with the LSU manually?

You should be able to add anything an orange module does with the LSU. 

I'd say you should be able to do it with the CAB2, but I ran across a situation that I had to resort to the LSU for, so I omitted that.

You can turn it off as long as you have the  legacy base1.51 software or later. See this thread: https://ogrforum.com/topic/s3-mars-light-help