[development] Inserting fields into a form

I'm hooking hook_form_MY_FORM_ID_alter in order to add additional fields to the form. The current fields have associative keys ('name', 'mail'). How do I add fields between them? There isn't a PHP call to do an insert into the middle of an array, and while using array_splice works, it won't use associative keys. The numeric keys result in the form looking ok, but when I ask the CAPTCHA module to give me a link on the form, it puts it right between the first field (associative key) and the first that I inserted (numeric key), which I think has something to do with the key problem. Is there an api call I've missed to let me specify where to add fields, or do I have to rebuild the form array in order to do it?