I need a script that adds specific sized crop marks around a selected object.

Illustrator uses points as is measurement units. If you wish to use other then you will need to use 'UnitValues' in your script… This should give you the basic idea of how to create a new box .25 inch around the current selection. The example is set to work with just the one item but you can easily change that if you wish. Using 'visualBounds' in many cases is a better option as it also accounts for stroke widths should you have any… Your code is JavaScript and you are going to be disappointed to find you can't call on actions with this language. So you are going to have to code the rest too… At least you have the start…

#target illustrator
 
function main() {
     if (app.documents.length == 0) {
          alert('Open a document before running this script');
               return;
     } else {
          var docRef = app.activeDocument;
          with (docRef) {
               if (selection.length == 0) {
                    alert('No items are selected…');
                    return;
               }
               if (selection.length > 1) {
                    alert('Too many items are selected…');
                    return;
               } else {
                    var selVB = selection[0].visibleBounds;
                    var rectTop = selVB[1] + 18;
                    var rectLeft = selVB[0] - 18;
                    var rectWidth = (selVB[2] - selVB[0]) + 36;
                    var rectHeight = (selVB[1] - selVB[3]) + 36;
                    var padBox = pathItems.rectangle(rectTop, rectLeft, rectWidth, rectHeight, false);
                    padBox.stroked = false;
                    redraw();
               }          
          }
     }
}

main();