I don't know why you might want to do this by script, but here it is:

Main();

function Main() {
    if (app.documents.length == 0) {
        alert ("Please open a document, select some text, and try again.");
        exit();
    }
    else if (app.selection.length > 1 || app.selection[0].hasOwnProperty("baseline") == false) {
        alert ("Please select some text, and try again.");
        exit();
    }
        
    var the_document = app.documents[0];
    var the_selection = app.selection[0];

    var the_dialog = app.dialogs.add({name:"Change leading in the selected text"});
    with(the_dialog.dialogColumns.add()){
        with(dialogRows.add()){
            staticTexts.add({staticLabel:"Change leading in the selected text by"});
            var increase_by = measurementEditboxes.add({editUnits:MeasurementUnits.POINTS, editValue:0});
        }
    }
    var dialog_result = the_dialog.show();
        
    if (dialog_result) {
        if (the_selection.leading != Leading.AUTO) {
            the_selection.leading = the_selection.leading + increase_by.editValue;
        }
        else {
            the_selection.leading = (the_selection.pointSize * (the_selection.autoLeading/100)) +  increase_by.editValue;
        }
    }
}

Written in CS5.5, Windows.

Warning: it's not totally tested — just a quick exercise in scripting before going to bed.

Make sure that Preferences > Type > Apply Leading to Entire Paragraphs should be off.

Regards,
Kasyan