Parsing a list of single numbers and number ranges
Since the whole exercise is a string-transformation problem, I suggest performing it using a regex substitution.

import re

def expand_ranges(s):
    return re.sub(
        r'(\d+)-(\d+)',
        lambda match: ','.join(
            str(i) for i in range(
                int(match.group(1)),
                int(match.group(2)) + 1
            )   
        ),  
        s
    )


I think that expand_ranges would be a more descriptive name than unpackNums.