How to add plugins to wordpress theme?

You can always include the files of the plugins in your theme functions.php file. Of course you should put it into some reasonable structure to not get your theme bloated with the files and the code :).

• https://stackoverflow.com/questions/6974006/wordpress-package-plugin-with-theme
• How to bundle a plugin with a theme, or vice versa

You can use code like this (used in Carrington theme) to load all the plugins included in the folder “plugin” within your theme folder…

/**
 * Load theme plugins
 * 
**/
function cfct_load_plugins() {
    $files = cfct_files(CFCT_PATH.'plugins');
    if (count($files)) {
        foreach ($files as $file) {
            if (file_exists(CFCT_PATH.'plugins/'.$file)) {
                include_once(CFCT_PATH.'plugins/'.$file);
            }
// child theme support
            if (file_exists(STYLESHEETPATH.'/plugins/'.$file)) {
                include_once(STYLESHEETPATH.'/plugins/'.$file);
            }
        }
    }
}

/**
 * Get a list of php files within a given path as well as files in corresponding child themes
 * 
 * @param sting $path Path to the directory to search
 * @return array Files within the path directory
 * 
**/
function cfct_files($path) {
    $files = apply_filters('cfct_files_'.$path, false);
    if ($files) {
        return $files;
    }
    $files = wp_cache_get('cfct_files_'.$path, 'cfct');
    if ($files) {
        return $files;
    }
    $files = array();
    $paths = array($path);
    if (STYLESHEETPATH."https://wordpress.stackexchange.com/" != CFCT_PATH) {
        // load child theme files
        $paths[] = STYLESHEETPATH."https://wordpress.stackexchange.com/".str_replace(CFCT_PATH, '', $path);
    }
    foreach ($paths as $path) {
        if (is_dir($path) && $handle = opendir($path)) {
            while (false !== ($file = readdir($handle))) {
                $path = trailingslashit($path);
                if (is_file($path.$file) && strtolower(substr($file, -4, 4)) == ".php") {
                    $files[] = $file;
                }
            }
            closedir($handle);
        }
    }
    $files = array_unique($files);
    wp_cache_set('cfct_files_'.$path, $files, 'cfct', 3600);
    return $files;
}

…then you use function cfct_load_plugins(); during the theme initialization.