Are Dynamically Created User Specific Directories Possible?

The easiest way to do this is to use session variables... any reason why you can't use them there? In PHP it's just a matter of adding session_start(); to have access to session variables.

Theoretically, if you really can't use session variables, you could assign an ID to CKFinder which could work as a token (ID is passed to the connector) - it should be hard to guess and regenerated each time user logs in. In config.xxx you would have write code that translates this ID into server path.

Which version of CKFinder are you using?

CKFinder should create user folder automatically even if it does not exist. However, it needs the right filesystem permissions to the folder where subfolders are to be created.
I have never used Postnuke, however I believe it must use some kind of session management system. session_start() works fine most of the time, however maybe you need to execute some special code first to have access to session variables and session_start() is not enough.
I've took a look at the source code of PostNuke and there is a SesionUtil.class.php in the includes directory which probably does some magic to protest session variables as much as possible, try asking on PostNuke forums how to access session variables from another directory.

This works great for 'creating' the folder the first time. In my case, I need to allow some users to change between accounts, and therefore change the folder that CKFinder uses. How do I re-set the session and reassign the folder? I don't want users to need to 'log out' and then login. They simply select an account from an select menu to 'enter' that account. I'd like to be able to simply reset the sessions used by CKFinder only. Thanks!

To be able to achieve anything you need, it is important to understand how the configuration file is constructed.
On the left side of the CKFinder interface, you have a set of folders ("resource types"), their definition looks more or less like this one:

$config['ResourceType'][] = Array(
      'name' => 'Images',
      'url' => $baseUrl . 'images',
      'directory' => $baseDir . 'images',
      'maxSize' => 0,
      'allowedExtensions' => 'bmp,gif,jpeg,jpg,png',
      'deniedExtensions' => ''); 

Note that by default, in the definition of each resource type, two helper variables are used: $baseDir and $baseUrl.
So, to point each resource type into different root folder, you can either change the 'url' and 'directory' attribute, or change the $baseXyz variables.
CKFinder does not remember these values in any way - CKFinder loads the configuration file each time a command is executed (upload file, load files/folders etc.) and if any of the variables used to construct $baseDir or $baseUrl has changed, CFKinder will use the new value.

In your specific case, you should create the $baseUrl/$baseDir variables based on some other variable that is changed whenever user change his account.