Adding a NetBackup client to a policy from the command line is easy using the bpplclients command.
bpplclients
Ex.
bpplclients my_test_policy -add my-test.mydomain.com Linux RedHat2.6
or for a Windows server
bpplclients my_windows_policy -add my-wintest.mydomain.com PC Windows2003
To confirm that it has been added you can run bppllist my_test_policy -U and review the list of clients in the policy.