Add specific word before the category page title

Since WP 4.4 you can change document title using document_title_parts filter hook, in earlier versions it will be wp_title_parts (since v4.0) or wp_title filter.

add_filter( 'document_title_parts', 'se333744_site_title' );
function se333744_site_title( $title )
{
    if ( isset($title['title']) && strlen($title['title']) && !is_front_page() && is_archive() )
        $title['title'] = ' {some prefix} ' . $title['title'];
    return $title;
}