To add custom field to the custom search form

Could you tell how to add custom field to the customized property search form.
For instance we can use with the mod_ospropertysearch the following for field property type:

(from mod_ospropertysearch.php) ...
$lists = JHTML::_('select.genericlist',$typeArr,'property_type','style="width:'.$inputbox_width_site.'px !important;" class="input-medium chosen" ','value','text',$property_type,'property_type'.$module->id); ...

(from default.php) ...<?php echo $lists; ...

Could you tell about the best way for custom fields?