Trezor itself (the firmware) already supports several altcoins and adding another is one line of code, providing that altcoin is derived from bitcoin.
I must stress though, that our dev team is reluctant to add just about any coin unless that coin is really supported / used by the community.

myTrezor.com is designed as a lightweight web interface to Trezor only supporting Bitcoin. To use Trezor with an altcoin - the altcoin wallet has to implement Trezor support + have a functional blockchain explorer.

Implementing Trezor into web/mobile wallets and bitcoin services is fairly easy with the TREZOR Connect API