Its not easily done. The cost of a larger generator would be much less than either an 'energy storage device' or modification of the appliances to reduce their start requirements.
You have to deliver the stored energy as AC power. This would require some sort of storage (battery or capacitor) plus an inverter.
Here's a thought: If your present generator can start and power a single appliance, arrange a set of switches or relays to connect one appliance to the generator at a time. Since a fridge and a/c don't have to run continuously, you could switch the generator between the two. Manual switches would suffice, if you are willing to monitor each units' t'stats. If you could do a little re-wiring in the appliances, you could have their thermostats signal some relays to switch the source back and forth with a couple of relays.
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