I personally believe a properly managed Mantis install should have versions added when a release is produced, so anyone reporting a bug shouldn't need to add a version. In addition, people with mere "reporter" access can't be trusted to enter version strings correctly!

However, as a developer I wanted a shortcut to quickly add a version when resolving a bug. The changes I made are as follows:

Mantis 1.2.0a3.

bug_change_status_page.php, approx. line 253:

<!-- Fixed in Version -->

<tr <?php echo helper_alternate_class() ?>>

<td class="category">

<?php echo lang_get( 'fixed_in_version' ) ?>

</td>

<td>

<select name="fixed_in_version">

<?php print_version_option_list( bug_get_field( $f_bug_id, 'fixed_in_version' ),

bug_get_field( $f_bug_id, 'project_id' ), VERSION_ALL ) ?>

</select>


<?php if ( access_has_project_level( config_get( 'manage_project_threshold' ) , $t_bug_data->project_id ) ) { ?>
or create new version: <input type="text" name="new_fixed_in_version" />
<?php } ?>

</td>

</tr>

bug_update.php, approx line 84:

$t_bug_data->due_date  = gpc_get_string( 'due_date', $t_bug_data->due_date);


$t_new_fixed_in_version = gpc_get_string( 'new_fixed_in_version', '' );
$t_new_fixed_in_version = trim( $t_new_fixed_in_version );
if ( !is_blank( $t_new_fixed_in_version ) ) { 
if ( !access_has_project_level( config_get( 'manage_project_threshold' ) , $t_bug_data->project_id ) ) {
access_denied();
}         
if ( version_is_unique( $t_new_fixed_in_version, $t_bug_data->project_id ) ) {
version_add( $t_bug_data->project_id, $t_new_fixed_in_version );
$t_bug_data->fixed_in_version = $t_new_fixed_in_version;
} else {
trigger_error( ERROR_VERSION_DUPLICATE, ERROR );
}

}
if ( is_blank ( $t_bug_data->due_date ) ) {

While I don't think letting "reporters" create versions is a good feature for Mantis, you could easily customise your own install using similar code to this.