Different show postbit username?

[Sinan COSKUN]

Hi,
I m sorry for my english
There is a question, pls help.
I want that users give a nick when they are registering
but they must be given their name and surname for the extra field (fieldX)
Their name and surname are show postbit.

Example:
username:1234
password: 5678
fieldX: Sinan COSKUN

user is enter forum:
use 1234 and pass:5678 but they are send post show postbit "Sinan COSKUN"

I do in this way:

postbit_legacy

<div id="postmenu_$post[postid]">
<if condition="$show['profile']">
<a class="bigusername" href="member.php?$session[sessionurl]u=$post[userid]">$post[field9]</a>
<script type="text/javascript"> vbmenu_register("postmenu_$post[postid]", true); </script>
<else />
$post[field9]
</if>
</div>

(to change $post[musername])
But group color same with member (admin, mod)
What can I do?