Presumably it's not built-in because it's rarely needed, the actual prefixes are much more common requirements. It can be easily scripted of course (I realise this is a feature suggestion but I'm just pointing this out in case you didn't know).

Code:
alias nick_modes {
  if ($2 ison $1) {
    var %r = $remove($nick($1, $2).pnick, $2), %i = 1
    while ($mid($prefix, %i, 1)) {
      var %r = $replace(%r, $ifmatch, $mid($nickmode, %i, 1))
      inc %i
    }
    return %r
  }
}

Spelling mistakes, grammatical errors, and stupid comments are intentional.