Hi Luluc,
If you want to modify a style to add a 'crash', or increase the volume of an existing one, then it would be easier to just use the keyboard's Style Creator to add the note to the Rhy2 channel, rather than use the 'Assembly' function to copy a different fill. This method retains the original drum pattern for the fill.
The Rhythm channels record in 'overdub' mode, so you just need to find the correct note to play for the crash cymbal, start recording the channel, and play the note at the appropriate time. This will add the crash to the existing pattern on that channel.
You could also add the crash, or increase the volume of an existing one via the 'Step Edit' display, after setting the Rhy2 channel to record.
However you would need to do this for every Fill, in every style, that you want to have the crash! This could be a lot of work!
A possible less onerous alternative could be use a foot switch/pedal (such as Yamaha FC5) to play the crash on demand!
To do this you would simply assign the 'Percussion' function to one of the pedal inputs.
When you select this function you will be able to select the drum kit used, and the sound to play. Once set, each time that you press the pedal, your selected sound will play, in your case one of the crash cymbals. You can then crash away whenever you like!
On Genos (unlike tyros and PSRs), the foot pedal settings are stored as system settings, so once set they will remain in place through power off/on cycles.
Regards
Ian