jquery e kullanmak istemiyorsaniz aşağıdaki gibi işinizi görür;

<script language="javascript">
var nereye = 'quantity';
function tusGir(tus){
	eval( 'document.numPad.'+nereye + '.value = document.numPad.'+nereye + '.value+' + tus );
}
</script>
<form action="" method="post" class="" name="numPad">
<table cellpadding="2" cellspacing="2" border="0" width="44">
<tr>
     <td colspan="3" align="center">
		<p align="right">Şifreniz
		  <input type="text" name="quantity" size="10" onfocus="nereye='quantity';"></td>
</tr>
<tr>
     <td colspan="3" align="center">
		<p align="right">Şifre2
		  <input type="text" name="quantity1" size="10" onfocus="nereye='quantity1';"></td>
</tr>
<tr>
     <td><input type="button" value="  1  " onclick = "tusGir('1');"></td>
     <td><input type="Button" value="  2  "  onclick = "tusGir('2');"></td>
     <td><input type="Button" value="  3  "  onclick = "tusGir('3');"></td>
</tr>
<tr>
     <td><input type="Button" value="  4  "  onclick = "tusGir('4');"></td>
     <td><input type="Button" value="  5  "  onclick = "tusGir('5');"></td>
     <td><input type="Button" value="  6  "  onclick = "tusGir('6');"></td>
</tr>
<tr>
     <td><input type="Button" value="  7  "  onclick = "tusGir('7');"></td>
     <td><input type="Button" value="  8  "  onclick = "tusGir('8');"></td>
     <td><input type="Button" value="  9  "  onclick = "tusGir('9');"></td>
</tr>
<tr>
     <td colspan="3">
		<p align="center">
		<input type="button" value="       0         "  onclick = "tusGir('0');"></td>
</tr>
</table>
</form>

eval kismi document.getelementby id ile daha tatli gorulur aslinda ama yapiniz ile ayni olsun diye boyle kivirdim :P