To expand a NWSSTG it must be 1025 MB or larger.
Use the command CRTNWSSTG new_name FROMNWSSTG(new_name) NWSSIZE(new_size). Then link the new space instead of the old space and restart the server.
The new NWSSTG space “drive” will have a formatted space equal to the size of the old space. The additional space will appear as unformatted space on the drive.
To expand the usable space to the capacity of the new NWS, you will need the VolumeManager product from PowerQuest (Partition Magic for Windows servers). This will allow you to expand the formated area to fill up the “drive”.
Unfortunately, this does not seem to be documented anywhere. I found it out by speaking to an IBMer at COMMON who was involved in the Windows Integration group
The other way in Windows 2000 and up would be to just make and link a new NWS space, then make the two “drives” in Windows into a single volume, so it appears to the users as a single drive.