Can you use an extention cord with a table top microwave oven?

Yes, and extension cord can be used with a microwave. But it is critical to pick an extension cord with a sufficiently high rating on it. Hardware stores and large home stores have extension cords specifically for use as appliance extension cords. They are heavy, and they have only a single place on the end to plug something into. They cost a bit, but are worth it in peace of mind.