D03 is an industrial standard for a subplate mounted valve.
In order to use a subplate mounted valve, you need to buy a subplate, which generally are not all that expensive.
Their claim to fame is that standard bar manifolds that hold anywhere from 1 to 10 sublate mounted valves are avaiable, some with auxilliary valves, like a relief valve included in the bar manifold. There are also "sandwich" valves avaiable that go between the subplate mounted valve and the subplate. All kinds of functions are available this way, counterbalance valves, flow controls, pressure controls, etc.
As pointed out earlier in this thread, they are not all that well suited for typical "mobile" applications.
The reason is, mobile open center valves typically have a parallel passage, which allows unused oil to go to tank. D03, D05 D08 etc industrial valves do not have this feature.
Since the way your current stack valve is plumbed, or not plumbed, as the case may be, with no access to the parallel passage (power beyond) you are kind of stuck.
As Mad mentioned, pressurizing the tank port is not a good way to go. Most valves are derated on the tank port, and you could be risking a failure. There are other potential problems as well.
You have two options, a 6 way selector as previously mentioned plumbed to the work ports of the existing valve, or you could plumb another 4 way three positon valve either in parallel with the existing valve, or in front of the existing valve.
If the new valve does not have a parallel passage, a two way valve in series with the existing valve would be required. this valve would need to be closed more or less simultaneously with activating functions on the added stack. Because of the "simulatneous" part, this pretty much means it will only work well with solenoid valves.
The way this is typically done is to use a double pole switch on the added functions, with the extra pole of each switch used to shift the four way, used to activate the 2-way, closing off tank flow through the original valve, forcing the oil through the new stack valve.
There are things to watch out for. Direct acting 4 way 3 pos solenoid valves typically can't handle all that much flow. Since reasonable wattage coils put out relatively low force compared to a human with a long handle, it doesn't take all that much flow before the sol valve gets stuck and won't shift. Also, you need to make sure there is a relief in the circuit when the two way is open or closed.
If the valve you add has a parallel passage, it could be plumbed "first" with power beyond from the new valve plumbed to the exisiting valve.
If it needs to be a solenoid activated valve, and you don't like the added two way and double pole switches, then there are at least a couple of manufactures of 4-way 3 pos. sol. valves with a parallel passage. Parker sells an old FPS direct acting 4 way 3-pos. solenoid valve that includes a parallel passage, but it craps out around 6 GPM, and is expensive.
Husco has a direct acting 5000 series section that has a parallel passage, can be banked with manual 5000 series sections, and can handle up to around 16 GPM, but it too, is expensive.
I'm not sure what Prince and the like have in solenoid valves with a parallel passage, but I'm sure a search could turn up something.
