[Simon Lloyd]

You can create a custom profile field, your users can fill it in and you can show it under the username like this:
In postbit (or postbit legacy) look for

<if condition="$post['usertitle']"><div class="smallfont">$post[usertitle]</div></if>
				<if condition="$post['rank']"><div class="smallfont">$post[rank]</div></if>

and under it add

<if condition="$post['field10']"><div class="smallfont">MY TEXT HERE: $post[field10]</div></if>

where "field10" would be the field number of your custom profile field