Create Something Like NTFS Junctions in Android 2.2
I think you may want a symbolic link.

You can create them in any good Linux system using

ln -s &lt;link_target&gt; &lt;link_name&gt;


eg

ln -s /data/app /mnt/sdcard/data/app


or (while in /mnt/sdcard/data)

ln -s /data/app app


You will want to be careful regarding removing the SDCard - you will need to completely shutdown the device prior to removal, and the same SDCard will likely need to be in the device prior to booting.