You could just rewrite /fopen, here is an example:

Code:
Opens the specified file and assigns a name to it for later reference
-n switch creates the file if it does not already exist, fails if the file does exist
-o switch creates a new file, overwriting it if it exists
-a switch opens a file if it exists, creates it if it doesn't

alias fopen {

  ;set up /fopen command to use mIRC's internal command
  ;also check to see if fopen has been called with a preceeding "." to
  ;not show it's results
  ;%fopen looks something like: !.fopen  
  var %fopen = $+(!,$iif(!$show,.),fopen)

  ;check to see if any switches have been specified, if so make sure they are valid.  
  if (-* iswm $1 && !$regex($1,/^(?:n|o|a)$/i)) {
    ;handle the syntax error how u feel
  }
  
  ;if the switches are valid, and the switch is -a
  elseif ($1 == -a) {
  
    ;check to see if the file exists, and if so, open it with /fopen
    if ($isfile($3-)) {
      %fopen $2-
    }
    
    ;if the file doesn't exist, open it with "/fopen -n" to create it.
    else {
      %fopen -n $2-
    }
  }
  
  ;if /fopen was called without any of the improvements we are trying to make, use the sent command as is.
  else {
    %fopen $1- 
  }
}

I haven't tested it or anything but the concept is pretty easy to follow

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