Hey Daniel,
You cannot do this from directly within the form editor, but you can do this with the addition of some custom code. Since Forms 2 loads asynchronously you'll need to use the whenReady event to make sure that the code executes correctly:
<script>
MktoForms2.whenReady(function(form){
$('#yourId').addClass('myClass myOtherClass'); //add a class
$('#yourId').attr('yourAttribute', 'yourValue'); //add an attribute with a value
});
</script>
Keep in mind that you can only have a single id per element, so overwriting this is not a good id, and can break form functionality.