macOS Seirra has this built in now, but older version of OS X can still make use of this.
1. Open System Preferences > Keyboard > Shortcuts > App Shortcuts
2. Click the plus icon on the bottom of the pane
3. Choose Finder from the pull down menu
4. Enter Compress in the text field
5. Enter a shortcut keystroke
6. 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.