I don’t believe there is a way to accomplish that with Airtable as it now stands. (One way to think of it is that what you are trying to do would require a field to be both a single-line text field and a formula field at the same time: The text field accepts user input, and the formula field appends " kg" to the entered value. Not allowed…)
What you could do would be to define two fields explicitly, the first (single-line text) for data entry and the other — formula, with a defined formula of
IF({TextField!="",{TextField}&" kg,"")
— for data display. Typically, you would tailor your Views to fit intended functionality, hiding the formula field in data entry screens and the text field on those intended for data display.
Please note currently there is no way of supporting both functions with a single field — for instance, having the user request a screenful of data and then editing it in place, changing, say, “136 kg” to “172 kg” — as that would require data entry into a formula field.