macOS Seirra has this built in now, but older version of OS X can still make use of this.
- Open System Preferences > Keyboard > Shortcuts > App Shortcuts
- Click the plus icon on the bottom of the pane
- Choose Finder from the pull down menu
- Enter Compress in the text field
- Enter a shortcut keystroke
- Click Add
You can use this trick for a lot of context menu items that don’t have a shortcut. You just need to exactly match the verbiage of the menu item being selected.