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.