I think the Mp3 is compiled code. Unless you can read and write in machine code I don't think you can modify the MP3 directly.
As to the Job step use the Jog flyout panel and set the set value to whatever value you want, you have a selection. You can also change the the values that the step jog values are chosen from in CONFIG.
If you are wanting seperate step jog buttons then use screen4 and add buttons to you screen then add the jog value code you want.
0.0001mm step

Does your machine axis resolution even support that small of a move? Mach can only move at the smallest step resolution of the axis
(;-) TP