Can I install an additional breaker bus bar on my main breaker panel?
The panel you have pictured is a grounded panel but originally designed for just 12 branch circuits.  There would be no problem if you added an extra ground buss as long as it is screwed securely to the metal box and bonded to earth ground with a piece of 10 or 8 AWG. 
Knowing you want to add kitchen circuits, looks like you have plenty of capacity. Minimum of two 20 amp convenience receptacle circuits (GFIC), one for the fridge, one for the dishwasher, and one for lighting.  Looks good to me. I'd pass it on my inspection.