Unfortunately there is no hook for above the fields (perhaps you might want to submit a patch/feature request on trac?)
You’ll have to get by with JavaScript:
<p id="klantnr-field">
<label for="klantnr">Klantnummer<br>
<input type="text" tabindex="20" size="10" value="" class="input" id="klantnr" name="klantnr"></label>
</p>
<script>
(function ( form ) {
form.insertBefore( document.getElementById( "klantnr-field" ), form.childNodes[0] )
})( document.getElementById( "loginform" ) );
</script>
And I hope you’ll be sanitizing that $klantnr
variable if you plan on using it 😉