stsadm cannot do what you are asking unfortunately, however you can achieve the same functionality using SharePoint Designer, which will create a .cmp (which is a cab file by another name) file which you can import using either stsadm or SharePoint Designer as you prefer.

Here are some links to SharePoint Designer as well:

 - [SharePoint Designer 2007][1] (works with WSS3 and SharePoint 2007)
 - [SharePoint Designer 2010][2] (works with SharePoint 2010, 2007 and
   WSS3(last 2 with reduced functionality))
 - [SharePoint Designer 2013][3] (works with SharePoint 2013)

A quick example on how to use SharePoint Designer 2007, to illustrate:
Initially go to `File > Open Site` and you will be presented with a request for either a local directory containing a site or a URL, enter the URL of your SharePoint site.

Once you have done you may have to authenticate, just follow the on-screen prompts, and fill in the details for your admin level account.

Now you can see the `Folder List` (far left) and `Web Site` (main part) windows are populated.  

Select a site from the Folder List (single left click) then navigate to `Site > Administration > Backup Website` whereupon you will be presented with a requester again which has a check-box to include sub-sites.  

Leave this un-ticked and Voila! you can export a site without its children.

Nice and straight-forward.


  [1]: http://www.microsoft.com/en-gb/download/details.aspx?id=21581
  [2]: http://www.microsoft.com/en-gb/download/details.aspx?id=16573
  [3]: http://www.microsoft.com/en-GB/download/details.aspx?id=35491