Can hifu therapy be used to lift your butt?

The quick answer is probably no, but try it and let us know if it works, it won't do any harm.  Probably just cost some money and time.

Hifu therapy has not proven to be an effective method for tightening sagging buttocks. The only way to tighten, lift, or reshape the buttock region is to either undergo fat transfer or placement of buttock implants or directly excise the excess skin.

HIFU will not lift the buttocks. Focused Ultrasound has been disappointing.To shape and lift the buttocks there are two ways1: fat transfer to the buttocks. or buttock Implants. This will increase the volume and lift.2: surgical Lift. Tighten the skin, but leaves a scar on the top of the buttocks.There is no non-surgical methods to lift skin, tighten skin that has lost elasticity.

You may want to look into VIORA, a radio frequency laser non invasive treatment that will tighten up your skin. A complimentary consultation would be necessary to make definitive recommendations.