You can't call script to be executed only, but you can configure script to be called as part of either pre or post backup operation via savepnpc.

ok cool i will checkout the admin guide

Can you give me more info?

In backup command you will need to put savepnpc and upon first run it will create .res on client. There you can specify pre and post commands along with some other parameters. It is pretty well explained in docs. Next time you run it it will call pre command, execute save job and then call post command.

You can use any customized script in the backup command field. This script should utilize save on the client. When you use the save program, a new instance of the customized script is invoked for each save set listed in the client¿s save set attribute, rather than just once for that client (as happens with savepnpc).
Note: The syntax used in the backup script or batch file must begin with either the prefix of save or nsr and the program must reside on the client in the same directory as the NetWorker save command.

Best regards,