Good question!, let me answer it in two parts, first a little background.....
It is not possible to do this out of the box as it is a security issue, any tenant admin could add another tenants machine and gain access to their data.
1. Manual Process (Easy), you could build a form (using the NSS forms engine) for users to make requests to add machines, this could be routed to an NSS admin (via approval) to carry out the task manually.
2. Automated Process (Not so Easy), you could build a form (which is similiar to the existing manual Add Machine form) which captures the information and then passes to the NSS API to add it. Clearly rules would need to be agreed on how this process works for each tenant, it may be that nobody cares if they add another tenants machine or some attributes are enforced via the form.
For 2. we have been requested this in the past and quoted 3 days of professional services to build and test it, you would need to go via your Veritas account manager if you wanted to engage us.
Hope that helps