Your #10 cable is limited to the 30 amp CB you have on it now. You' ll need a complete run of new cable, properly sized for your 100 amp service to the new panel.

You can put a 100A panel in if you want, but you certainly can NOT increase the size of the breaker that's protecting the #10 feeder beyond 30A.

You can definitely not tap the main lugs however for a feeder to a second building.

Even if you are not exceeding the 30A you're probably starting to get into voltage drop issues at ~100' that probably want you to size up the conductors anyhow.

Randy is right, you really want to pull a sufficiently sized feeder to your new structure.

Note these structures require grounding systems.

I appreciate your input. . I think now I am clear on what I need to do.

Thanks again.