1- First of all you can see external page of site. There is some ideas about doing that.
2- Then note that if you include qa-base.php, then you can run q2a functions when include appropriate headers. for example:
include '[path to include dir]/qa-base.php';
$modules = qa_list_modules('module');
prints all modules names.
Now, you can create separate files for question post, add user, ... that you want to use in other product