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You would have to import your Metamask seed into Trezor and then delete it from your PC.
This is inherently less secure than using a Trezor-generated seed — for all you know, malware has already stolen your seed and the attackers are just waiting for a good time to drain the account.
But it is still more secure than just keeping the seed in a hot wallet, because any future malware could still steal it.
Remember that for any security gains at all, you must make sure to delete the Metamask account and erase any other traces of the seed from your PC.