I just set up my Trezor 3. It has several coins already listed but how do I add new accounts not listed? I want to add XLM but there is no option to add anything not already listed and there are only like 6 coins listed.
There are several types of supported coins in Trezor devices. Some of them are natively supported in Trezor Suite, and some of them are not. For those, you need to use compatible 3rd party wallets. You can find more information in this video: https://www.youtube.com/watch?v=w-3XRO5u8kc&t=1s
You mentioned that you are interested in XLM which is the cryptocurrency that is supported in Trezor devices but not natively supported in Trezor Suite. It means that you need to use 3rd party wallet (e.g. Account Viewer for XLM) to use this cryptocurrency with Trezor device. More information on how to connect Trezor to Account Viewer can be found in this post: Receiving stellar XLM to trezor 1 thru trezor suite - #4 by radekP