While it may not be feasible to build modules using cake, and i don't think i would advise it anyway, it is more than possible to very easily use your cake output in your CMS-MS site.
custom usertags allow you to get the cake processing easily
include('
http://yoursite/cake/controller/action/params')
call anything you want.
oh, you want to actually get the data that cake creates? well, CMS-MS just becomes your view in that case.
in your cake controller, instead of '$this->set'ting your variables
serialize the results and echo them at the end of the controller action and exit so no render is performed.
in cake controller, in whatever action:
$data = $this->Model->findAll();
echo serialize($data);
exit;
in your custom tag in CMS-MS use this php:
$data = file_get_contents('
http://yoursite/cake/controller/action');
$data = unserialize($data):
bingo.
i prefer to set my layout to null in cake, and just echo the formatted results with cake's views and not worry about manipulating variables in CMS-MS.
Now, i am sure there is a way to create a helper of somesort, in cake, that uses CMS-MS features in depth, but i didn't think it worth the hassle.
i just setup the admin of my cake side to use the same auth tables as CMS-MS