Copying a part from a different section of the same style is covered in the video starting at 1:32:57.
In the example, he actually copies the Rhythm2 part from different sections of the same style, but I'm sure that even a 'house plant' could work out how to copy the Bass part instead!
I'm not sure how Genos addresses the situation where the measure lengths of the original section and the section to be copied are different, so this may be something to consider if you get unexpected results!
I agree about the quality of the information about this function in the Genos Reference Manual. It is really poor, even by Yamaha's low standards!
One thing to remember is that you must select a style section of the loaded style to copy the changes to by pressing one of the style section panel buttons. The changes that you then make in the assembly process will be to this section.
Although the name of the style that you are changing appears at the top of the Style Creator display, it seems to be too much trouble for Yamaha's shoddy programmers to actually indicate the currently selected style section as well! Although you can see this from which style section button is lit orange/red, it wouldn't hurt to show it on the display too!
Regards
Ian