Need help and advise on valve adjustment with non adjustable rocker arms & shaft

You could. Just like the rocker shaft shims. But at least in my opinion it's not the right way to do it. Assuming the caps fit your retainers, you'll end up with the rocker pushing the valve stem more sideways than it should as it goes thru the lift cycle, and will end up with faster guide wear. If the geometry is already not great, it can make the valvetrain less stable, leading to lower rpm potential and power too.