Usually it is believed that NTFS directory junctions are not allowed to link network shared folders.
Refer to this MSDN article: http://msdn.microsoft.com/en-us/library/aa365006%28v=VS.85%29.aspx
However here is a easy workaround:
1. Create a symbolic link to the network shared folder
2. Create directory junction to the symbolic link
The link created in step 2 looks like a directory junction, but behaves like a symbolic link.