Could you either parse the text or add a syntax for converting numbers(kg/lb, liter/gallon, etc.) between metric and imperial? Possibly by showing the original input, but if I hover it with the mouse, it'll display it in the alternate scale.
Simply parse the numbers as written, though this can be harder, and may have undesirable side effects.
Simply allow people to wrap measurements in brackets, like [1kg], and it'll get parsed into a way that displays both 1kg and 2.20lbs.