Rangemap only takes explicit integers. However, rangemap basically only is shorthand for case:
... | eval range = case(field < low_threshold, "low", field < elevated_threshold, "elevated", field >= elevated_threshold, "severe")
Note, this is not exactly the same as the first rangemap you quote - negative values get the default from rangemap but low from this case. Just take care to specify the case conditions accurately to match your requirements.