I'm not aware of any way to do this in ImageJ. When I need to plot more than one data set on a single graph, I save the values and then bring them into excel where I can create a the graph. Just click the "List" button in the plot and then copy and paste the values into Excel.
If you use the "Data >>" button after obtaining your plot profiles, a drop down will appear. You can select the "Add from plot" button which will let you add other plot profiles currently pulled up onto the one you are using. From there you can change colors of the plots being added in.
, sorry that I am just now seeing this. You are correct, I have the FIJI version of ImageJ and did not even think about that. I just had assumed the option was available in both programs.