SourceTree uses the default “Open With” application like Explorer. As far as I know other than changing the default application used by each file extension you cannot change the behavior of the menu item in SourceTree.
However…
You can add a custom action to achieve the same result using Tools | Options, Custom Actions tab:
This will appear under the Custom Actions menu. As far as I know you cannot add a shortcut key to custom actions.
The SourceTree blog has more info.