Instead of adding clients manually one at a time through the Networker Management Console GUI, you can perform an initial bulk import using cmd line interface:
nsradmin -i import-file
where the import-file contains many lines like this
create type: NSR Client;name:win-vm;comment:SOME COMMENT;aliases:win-vm,win-vm.enlit.me;browse policy:Day;retention policy:Day;group:VM-Image-Daily;Proxy backup host:my-vcenter.enlit.me;Proxy backup type:VADP;
Use excel to form a large csv, then use Notepad to replace commas with semi colons. Be aware there is a comma in the aliases field, so use an alternative character in excel to represent this then replace it with a comma once all commas have been removed from the csv.
If you want to add additional attributes, just check an existing client entry. The attribute name is usually same as the one displayed in client window.
nsradmin> show name ; group; browse policy; retention policy;
nsradmin> print type: NSR client; name: jwin-vm.enlit.me
schedule: Daily Incremental;
browse policy: 3 Days;
retention policy: 3 Days;
A list of other attributes can be found here too: http://nsrd.moab.be/nwcli8/nsr_client.html
More command line options can be found here