Create globally available volume control for iTunes
The best resource for anything like this is Doug's AppleScripts, in this case possibly the collection by Wally - Wally's Scripts v1.0 

There is also a simple tutorial on how to make your own - Setting iTunes EQ and Volume with AppleScript

The only extra thing you need is to make these using Automator, as a Service, then use System Prefs > Shortcuts > Services to assign them global hotkeys [making sure to avoid other used key commands]