Probably the best way is to create a custom New User Profile Field in Admin CP. Either a single or Multiple-Line Text box, depending on your needs. Adjust the maxlength from "100" characters to a number you desire. Set all remaining fields and be sure to make it "Editable" for your users. Remember the "Field ID", as you'll be needing that in a bit.
Once completed, in your postbit or postbit_legacy find:
Code:
<div>
$vbphrase[posts]: $post[posts]
</div>
And under that add:
Code:
<!-- Custom User Profile Field -->
<if condition="$post['fieldx']"><div>
$post[fieldx]</div>
<!-- / Custom User Profile Field -->
Where "x" is replace that with the Field ID # you just created. When users fill this Profile Field in User CP it'll show up in their post user the post count.