You cant do this in FL Studio (yet?), but you can take a look at this thread, it should be of interest to you:
http://forum.image-line.com/viewtopic.php?t=59320
Multiple channels' volume tweaking
Return to “FL Studio Users Forum (Looptalk)”
Forum rules
Please read them here.
Please read them here.
[You can only see part of this thread as you are not logged in to the forums]