Here is a small guide how to add apps, which you actually canīt find with the marketplace search function of the folder app, to an existing folder:
1. Download Windows Phone Power Tools from here: http://wptools.codeplex.com
2. Install and run it on your desktop
3. Go to "file browser"
4. Search for the guid of the folder app (a2e30887-7682-43cf-9a71-c20104b190fa)
Note: if you have installed the folder app with Windows Phone Power Tools you will find "Folders" instead of the guid
5. tap on the guid or on "Folders"
6. you will see a file type xxxx.folder (xxxx is the name of your before created folder)
7. Press "Get" button and save the file to your desktop or just use dragīn drop function
8. Open this file with the text editor and add the name, the guid and the iconname of the app you canīt find in the marketplace (just check the existing apps and you will see how to do that)
9. save the modified file
10. copy the xxxx.folder file together with the appicon (important: .jpg format) back to the device ("Put" button or just dragīn drop)
11. run Folders on you device and you will find the manually added app in your folder
12. pin the folder
Attached is an example of a self-created "Nokia" folder including the appicons, just copy, as descibed above, all files with Windows Phone Power Tools to the Isolated Storage of the folders app and voila you will see an Nokia folder...