You could buy a separate 100 amp main for the new panel but it would cost almost as much as the panel.
If you go to HDepot you can buy a 200 Amp Homeline panel with a number of breakers. Additional breakers are as cheap as any you can get.
Run some appropriate size wire for 100 Amps (2 hots, neutral, and ground) from the existing 100 Amp panel to the new panel, connecting to the lugs of the 100 Amp panel AFTER the main breaker. Connect to the standard input of the new panel. If there are no lugs you can probably buy a cheap 100 amp branch circuit breaker for the old panel on the famous auction site.
The new Homeline panel will have separate terminal strips for neutral and grounds, which you must use. The neutral should not be grounded at the new panel. Leave out the big green screw that they will furnish.
You can leave existing circuits in the old box or move them to the new panel. You can use the old panel as a splice box if necessary.
If there is adequate space you can connect the two panels through a short conduit, or multiple conduits. For the 15, 20, and 30 amp circuits with THHN wire you can run as many as 9 current carrying conductors in a single conduit. You can run more if it is a very short run.
If you ever upgrade to 200 Amps (such as if you need it for "grow lights" for yourpot plants) you can eliminate the 100 amp panel and connect the new service to the 200 amp panel. If you are going to do it to code you will need to provide grounding adequate for the 200 Amp service.