Hello vadesriux, sorry not to answer sooner, had some issues.
I think, and my understanding is, it's not posible to record volume or pan adjustements while recording, not even over a midi file done or terminated (editing), when you start a midi recording session, one of the first lines in the midi file are volume, pan... etc, these are maintained and stablish at the beginning with the setup values, you selected, and they are not changed in the middle of the session using the equalizer. When editing over a finished midi file, you can adjust these parameters one by one, but manualy, if you need to make adjustments like fade in or out (eg. volume) you can do it but you will have to include in the midi file, line by line with its corresponding value. The Yamaha keyboard editor for PSR SX900, is not a professional app, it only permits you adjust and incorporate easy things!, The adjustments you want/need, must be done with a professional DAW software, depending on your needs and expectations.
A very good and free software used by several members of our forum is "Mixmaster", I recomend you to search over the forum, take a look about it, download it and probably it will help you a lot when you need to adjust certain parameters and manage the kind of things you want/need to do with midi files.
Another very useful yamaha app for iOS, (don't know if there is a androide version) is MusicSoft, it permits to manage (import/export) all your files between the keyboard and your iphone/ipad/mac, whith or without cables, no need to use USB memory.
Regards
Bernardo