That would be a single spool open center valve, 3 position, spring centered to neutral. Your grasp of how it works is correct, but it depends a lot on the hoe valve capabilities and how your hoe valve is plummed into the system. Most valves with just a single input and return line as you have described are not rated for much pressure on that return line. It is typically supposed to take a low restriction path back to the reservoir. By putting a valve on that return line, you could cause excessive back pressure to parts of the main hoe valve and do damage to it(usually spool "O" rings).
A place where you could put your single spool valve would be on the input line. This side is rated for high pressure. You would need a single spool valve with "Power Beyond". Power Beyond is simply a high pressure path thru a valve from the high pressure inlet. A power beyond valve has 3 main lines. A high pressure Supply inlet, A High pressure PB outlet and a low pressure return. The return line is the fluid comming back from a moving cylinder or from the relief. Any back pressure on this line reduces the force applied by the cylinders so it must be a unrestricted outlet to the reservoir.
The input that now feeds your hoe valve would instead go to the new single spool inlet. The hoe valve input would then be connected to the PB outlet of the single spool. The Return line could be "T" d into the hoe valve output providing it is a low pressure return to the tank.
Another option, depending on your valve make, might be to get another section for your hoe valve and add another spool/lever to the stack for this function.
Yet Another option would be to get either a manual or electric dual selector valve.
Surplus Center Item Detail This valve has 6 ports. 2 connect to the work ports on your hoe valve. 2 connect to the cylinder that used to be connected to the hoe valve where we connected the first 2 ports. The last 2 go to the new cylinder. This would allow one of your existing functions, such as one of the outrigger arm levers, to switch functions with a simple lever movement and function as the thumb control untill switched back.
You might get more answers if you start this as a new topic in the hydraulics forum.