One way to look at the variables that are created in the workspace, is to first enable the parameter “Load Matlab” in the Matlab component. Then after the simulation you can write “workspace” into the Matlab Command Window which will appear.
It is possible to choose and then combine desired modes in a Matlab component by simply deleting parts of the input data structure. However, using OptiSystem’s built-in Mode Selector component may be even simpler. I have attached an example of using the Mode Selector component to capture and combine the 3, 5, 9 and 12 modes of a multimode fiber.