I have not heard of NetSuite selling just storage. If you bring it up to them they will most likely require a tier upgrade
I will say in my experience I’ve never seen NetSuite enforce anything because of file storage. I’ve seen accounts with 10x the limit on storage and NetSuite doesn’t mention it
I think they just use it as a “bargaining chip” at renewal to put more pressure on companies to upgrade
You may want to look into some external storage options. We’ve built a nice tool with Google drive and even onedrive in the past. There are others to like Box or Dropbox that integrate with NS