You're welcome.
Not all Backup software supports CAB extensions. Some do such as SnapProtect/CommVault/Veritas etc. It all depends upon the Backup Vendor's integration with NetApp. But, that's not the end of the game, you can still backup data using node-scope, but then you need to ensure that 'Physical Nodes' are added as Backup Client instead of SVM so that the backup data path is local.
What is a CAB extension?
It is used to setup data connections directly to the physical nodes which contain the data. Which means if the Backup Software supports NDMP CAB Extension, then you can simply add 'SVM' as Backup client and backup volumes without worried about which volume is sitting on which Node and if the volume is moved to another Aggregate/Node then 'CAB' extensions automatically informs Backup software about it's new location therefore the backups don't fail.
Without CAB extension: Backup software is not aware of the cluster physical resources, in other words when you add 'SVM/VSERVER' (using SVM-scoped as mentioned in Veeam documentation) which is a virtualisation layer with volumes & LIFs, backup software cannot make a decision about the locality of the volumes, that means if data connection is made on the NODE-1 then only the 'volumes' attached to the 'NODE-1' will have local path to backup-device(tape/disk), however if the 'volume' to backup is sitting on NODE-2, then it has no intelligence to switch data connection to NODE-2, rather it will make it a 3-way backup, from NODE-2 to NODE-1 to Backup-device. For this reason it is better to backup data using 'node-scope' so that the backups are local and simple.
You can use the 'ndmpuser' that you created to add the NetApp physical Nodes as well.
I have never worked with Veeam so have no idea about it's GUI so can't really guide any further, but I am sure you should be fine.
Thanks!