<p>Is there any way to enforce a template in Bugzilla to guide users fill in bugs descriptions ?</p>

<p>Actually, i'd like to put some markup texts in the bug description field and avoid the creation of custom fields.</p>

<p>I've installed version 3.2rc1.</p>

The mechansism described under
6.2.5 Particular Templates
(under the section called bug/create/create.html.tmpl and bug/create/comment.txt.tmpl) works pretty well for us.  Even though you say you don't want to create custom fields, adding some arbitrary HTML is easy enough.