XCM transport fees are currently limited to using DOT (or some other single asset configured by the runtime).
There is work planned under Asset Transactor on Asset Hubs not Working With Sufficient Assets · Issue #3958 · paritytech/polkadot-sdk · GitHub to give the ability to hook in asset-conversion for XCM transport fees, and thus be able to just-in-time swap whatever for DOT, thus completely supporting “fees in USDT” on Asset Hub.
Exploring options for other chains here: Asset Hub Brainstorming Session - Call Notes and Discussion Continuation - #2 by acatangiu