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 (works with WSS3 and SharePoint 2007)
- SharePoint Designer 2010 (works with SharePoint 2010, 2007 and WSS3(last 2 with reduced functionality))
- SharePoint Designer 2013 (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.