But imho this would require a "persistent world" dll and a complete set of vessels, base, factory that work with the sames rules. A huge work.
Well "persistent world" would be needed if you want online playing or so, just saving the current state of a mission and returning to the saved state would be pretty much easier. Building the buildings, bases, etc would be more complex, but if you already add provisions for the core operations in the cargo add-on, it would at least be simpler to produce them, then to start with a clean sheet design for any more complex world add-on.
Same rules is a different thing though. I don't know how much you want to go into the details for your add-on, I suspect KISS is still the target.
Practically, you can already do some very complex work by having the type of goods and the mass. More complicated strategy games had one or more quality attributes, but I can think of at least one way that saves you from having to include this. Querying the extended attributes of a good by name from a economy plug-in would be possible. Making a DLL which uses a SQLite database for storing and reading such attributes would be rather easy.
If developers could agree on a tiny standard interface to link into vessels (Of course, usually it is your standard which people use happily), which could be used for accessing different economic models and switch them by the taste of the player, would be a good long-term solution - I doubt many people will want to develop such stuff, even more people will want to use what ever is there.
As I see it, having the attributes type, meta-type (Liquid, gas, bulk, ...), amount/mass and maybe health/damage for the cargo would be already allow a lot of complex interaction. Most PnP RPGs don't go further.