Creating a detent action is purely mechanical, but you'd still need a different spool (longer on the ends) and then be able to add detent ends on the valve body. If it's a simple valve, likely that it doesn't have provisions to add different ends to the block even if you could locate and swap a new spool in.
Self-canceling is a combination of mechanical and hydraulic action, and I know that definitely couldn't be converted. The valve would have to be plumbed different internally --- needs a feedback loop and ports to create the canceling action --- so I doubt very much if you could convert an existing valve. There would have to be a lot of extra machining inside the block for the self-canceling action and/or ports for an external feedback loop.
Given the complexity to all of this, I doubt any manufacturer makes valves to be converted on both fronts. They likely just sell different models with all the specifics worked out unique to the valve block and spool used.
