That's simple enough...
In your header template add the following code at the very end:
Code:
<if condition="$bbuserinfo[posts]<10">
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="alt1">
Type your message contents here.
</td>
</tr>
</table>
</if>
Using that code you can address people in any user state or show different messages to people who have posted under 10 posts and people who have posted over 20, 30, 100 or whatever your like.