How are your clients going to upload the files? ftp or via a web application? Maybe a web application is a best option. I can't think of one at the moment, but I once use egroupware and that had a file manager module for users to manage files. It really depends on your needs and what are you trying to do.
A client can have several ftp users and all of them would belong to the same group. I am knew in ISP Config 3, so I may be wrong.
You can always create another group and add the users to that group. Take a look to the following commands:
However I am not really sure what you want to do. If you have huge new partition or hardrive, you could mount that partition in /var/www
That way all your sites and data from the sites (except mysql that is in /var/lib/mysql) would be store in your big partition. That way for ISPConfig everythig is normal. However, you would need o do something like this.
Rename the /var/www directory to something else (ex /var/www-back
Create a new /var/www directory
Copy or rsync the files back to /var/www that is using your new partition
Save the new partition configuration in /etc/fstab to check that everything is ok.
Depending on the amount of data you may have a big or a little downtime. This are just some ideas I am thinking on while I am writing. Let us now how it goes.