Check the branch breaker space ampacity, most would be good for 100A. You could change out the panel to a pass through type and then connect to the second lugs, or install a disconnecting device (probably fusible) ahead of the original panel and feed all off of it (see the specific sections - this isn't as good an option as it would seem).
You might want to check your service. If you haven't got the ampacity coming from the power company, why waste you money? Don't pay for service larger than you need or have capacity for.
