How to add additional field for group search?

i am follow this http://www.boonex.com/unity/extensions/entry/FREE_MOD_DeeEmm_Group_search_by_country  By: DeeEmm

to add the additional field for filter in group search.
but

function actionSearch ($sKeyword = '', $sCategory = '') {
parent::_actionSearch ($sKeyword, $sCategory, _t('_bx_groups_page_title_search'));
}

it only support $sKeyword & $sCategory ?

how i can add other parameter to do filter

thank you.