A typical valve set up consists of an input section, an output (return) section, and one or more control valve sections. Additional control valve sections are available from many manufacturers. Your dealer may be able to order single sections. Places like Northern Tool supply all sorts of hydraulic valve parts.
I haven't done it, but I wouldn't expect the job to be very difficult. The biggest problem likely would be adapting the mounting plate or finding a new one. I may do this myself as soon as I fed up grabbing the loader bucket curl circuit every time I need an additional rear circuit.
If additional control valve sections aren't available, an alternative might be to hook a new valve assembly in series with the existing one. I believe that open centre valve assemblies can be hooked up successfully in series. Some valves work for either open or closed centre systems and are supplied with plugs for closed centre operation. The plugs have to be removed for open centre operation.