How to connect existing MEW to Trezor?
You cannot import your existing MEW to Trezor wallet.

It is the security of a hardware wallet which makes it worth the price, and that is why the wallet is generated in Trezor offline.

Thus, the only way is to create a new wallet in Trezor. Transfer all your coins from existing MEW to Trezor. And after that, you can use Trezor with MEW to send or receive other alt coins which Trezor don't have support directly.

Hope this answers your query.