To get that separated with an underscore: $regexp('(0345min)',(\d\d)(\d\d),$1_$2)
leads to (03_45min)
Alternatively use an action of the type "Replace with regular expression" for the field
Search string: (\d\d)(\d\d)
Replace with: $1_$2
But the numbers that you apparently refer to, aren't a fixed string like "(NORM)".
So instead, you look for a pattern (and not a string that stays as it is) which consists of 4 digits, described by
which you group into 2-and-2 digits. And then you insert the separator between the groups.