1) open your xbmc version of VC and click edit
2) drag the voicecommands.xml file from the itunes edition on to the xbmc command tree
3) edit out the redundancies and make sure your commands don't conflict.
alternatively you could open the voicecommands.xml files in the command 'bin' (on the right of the command tree) and then drag what you want over to the active command tree. You can drag groups or commands over, but I just noticed that there is a bug. If you drag a command from the bin onto a newly created group it doesn't work properly... I need to look into that.
It is possible to turn groups on and off, so that you can use the same phrase for two different commands (i.e. use play for either xbmc or itunes) but it's a bit tricky. In the future I hope to detect which program has focus and automatically turn command groups on and off accordingly.
For now you could create a command like "switch to itunes" and then turn your itunes groups on and your xbmc groups off, if you want.
look under voxcommando commands for enablegroup and disablegroup
I am currently using eventGhost to do this for me automatically because eventGhost can detect which program has focus and send enableGroup and disableGroup commands to VC for me.
You can also use enableGroup* and diableGroup* which uses implied wildcards to match groups. So you could have a bunch of different XBMC groups and turn them all off with disableGroup* XBMC
