There's nothing built-in, to give you that functionality.
You can't hide the contents of folders outside of the sandbox, without actually blocking those folders from being accessed at all by the sandboxed program(s) with a ClosedFilePath setting:
Sandbox Settings > Resource Access > File Access > Blocked Access
You can block access to files or folders, and limit the setting to a particular .exe program or to all .exe's.
This isn't likely to work very well if the program is installed outside of a sandbox, and you are just running it sandboxed from that location.
If I install a program inside a sandbox, that already exists outside of the sandbox, I sometimes do some temporary renaming of folders.
I temporarily rename folders that are outside of the sandbox while installing the program inside a sandbox.
The sandboxed install won't see that the folders it is creating inside the sandbox, already exist outside of the sandbox, so it will create new files in the sandbox instead of copying those files that are outside of the sandbox.
Of course, after the sandboxed install, you would want to go back to the folders you renamed and change them back to their original folder names.
This doesn't totally eliminate copying of files from the unsandboxed folders into the sandbox, since the sandboxed program will still be able to see the contents of folders that are outside of the sandbox.
However, if the sandboxed program modifies those files, the modified version of the files will stay in the sandbox and will be the ones that are used by the sandboxed program after that.
This is part of a thread about installing Firefox in a sandbox, and have it create a profile with a new name inside of the sandbox, and not think that it has the same Firefox add-ons that already exist outside of the sandbox:
http://sandboxie.com/phpbb/viewtopic.ph ... 85&start=5