[Frosty]

This is quite easy, but you'll have to create a new profile field for this.

Go to:
User Profile Field Manager - Add New User Profile Field:

Title: Website
Description: Enter the link of your Website.<br />
<p>Please make sure that you don't add the:
"http://www.", or "http://". Just enter:
"YourDomainName.com". <br />
You may enter <b>one</b> domain name only. </p>
Profile Field Category: (Uncategorized)
Default Value: leave blank
Max length of user allowed input: 15
Field length: 25
Profile Field Type: Single line text box
Field required: No
Field editable by user: Yes
Private Field: No
Field searchable on members list: No
Show on members list: No

Save it..

Go to postbit or postbit_legacy, find:
$vbphrase[posts]: $post[posts]

Posts are in the div tag, and leave 'em in it. After the </div> close tag, add this code:

<if condition="$post[field13]">Website: <a href="http://www.$post[field13]" target="_blank" rel="nofollow"> $post[field13]</a></if>

Replace number "13" with the number of your field, as you can see on this screenshot:
http://i.imgur.com/LPuAk.png
Field number is shown in the overview of the profile fields, enjoy!

Screenshot of how it looks like after you have added it:
http://i.imgur.com/aPUf8.png