A MEL script for generating a frame padded number. In this case the source value that is frame padded with leading zeros is the start frame value from the Maya Render Settings window

padded_frame_number.mel

// Add zero digit padding to an int value
global proc string pvr_zeroPadding(int $num, int $padding){ 
  // Tip from the blog post: http://ldunham.blogspot.ca/2012/01/melpython-adding-number-padding.html

  int $lengthNum=size(string($num));
  string $padString; 

  if($lengthNum<$padding){
    for($i=0;$i<($padding-$lengthNum);$i++){
      $padString=$padString+"0"; 
    }
  }
  return $padString+string($num);
}


// Get the current frame padding level from the Maya Render Settings window
int $padding = `getAttr "defaultRenderGlobals.extensionPadding" `;

// Get the start frame from the Maya Render Settings window
int $startFrame = `getAttr "defaultRenderGlobals.startFrame"`;

// Generate a frame padded number as a string
string $paddedFrame = pvr_zeroPadding($startFrame, $padding);