It certainly can be done - it is a matter of writing the custom functions to do it. You can check the sample program: "SetTimCtr_SV.PC5" program in the "samples" directory which allow you to use a few push buttons to change 3 timers and 3 counters set value. There is no limit to how many variables can be changed in this way as long as the program memory is sufficient to fit in all the code.
Remember that you could use a potentiometer to connect to any unused analog input to form a good input device for changing variable values too.
