i want to post a current cookies session to the web server instead of sending the POST login stuff , how do i build the string form what i currently have

sprintf(body, "vb_login_username=%s&vb_login_password=%s&cookieu ser=1&s=&do=login\r\n", "myuser", "mypass");
sprintf(headers, "POST /login.php?do=login HTTP/1.0\r\nHost: www.somesite.com\r\n"
"Content-Type: application/x-www-form-urlencoded\r\nContent-Length: %i\r\n\r\n", strlen(body));

i am using a firefox addon to get the cookie session , so if i just send that it should not need to ask to login...