Use a variable. $findfile is extremely inefficient when you know the location already.

alias nicklist {
  var %path path/to/Nicklist.dll
  if ($1 == -al) { noop $dll(%path,ListBox,$window($2).hwnd Aline $calc($3 - 1)) | return }
  if ($1 == -rl) { noop $dll(%path,ListBox,$window($2).hwnd Dline $calc($3 - 1)) | return }
  if ($1 == -il) { noop $dll(%path,ListBox,$window($2).hwnd Iline $calc($3 - 1) $4) | return }
  if ($1 == -cl) { noop $dll(%path,ListBox,$window($2).hwnd Cline $calc($3 - 1) $4) | return }
  if ($1 == -dc) { noop $dll(%path,ListBox,$window($2).hwnd DColor $3) | return }
  if ($1 == -rs) { noop $dll(%path,ListBox,$window($2).hwnd $3 $4) | return }
  if ($1 == -s) { noop $dll(%path,ListBox,$window($2).hwnd $3 $iif($4,$4,)) | return }
  if ($1 == -ti) { 
    if (!$3) { return $dll(%path,$window($2).hwnd TopIndex) }
    else { noop $dll(%path,ListBox,$window($2).hwnd TopIndex $3) }
  }
}

It can be simplified more anyway if you set %arguments and then just execute that as in:

alias nicklist {
  var %args
  if ($1 == -al) %args = Aline $calc($3 - 1)
  if ($1 == -rl) %args = Dline $calc($3 - 1)
  if ($1 == -il) %args = Iline $calc($3 - 1) $4
  if ($1 == -cl) %args = Cline $calc($3 - 1) $4
  if ($1 == -dc) %args = DColor $3
  if ($1 == -rs) %args = $3 $4
  if ($1 == -s) %args = $3 $iif($4,$4,)
  if ($1 == -ti) { 
    if (!$3) %args = TopIndex
    else %args = TopIndex $3
  }
  if (%args) noop $dll(path/to/DLL,ListBox,$window($2).hwnd %args)
}

By the way, I don't get $iif($4,$4,).. The only thing that would do is replace 0 and $false with $null.