I've done it more than once to integrate zend libs in other non-zend projects.
Autoloader is not suggested for just inclusion of some libraries as it involves in worse performances (see zend reference about |end_Loader for that).
The best way (from both clear code and performances point of view) is very simple:
1) set the include path: (necessary or you'll have fatal inclusion errors):
set_include_path(implode(PATH_SEPARATOR, array(
'/',
get_include_path(),
)));
2) do a "require_once" of the library/ies you need, following the structure Zend/
e.g:
require_once "Zend/Mail.php";
//you can use now Zend_Mail* classes
note1: you don't have to place a "require_once" of all the needed classes, the main included class already do a require_once of the dependent classes.