1) The Harpoon cannot be added to the P-3 by modders, unless there is someone out there much more clever than the rest of us.
2) All kinds of capability can be added to the individual weapons that are already in the game. Amizaur's SS-N-27 Two Stage ASM is a perfect example. In the original version of this missile, it included a pop-up phase, as well as evasive maneovering. These advanced features were removed to make the missile easier for the AI to use and more reliable from a scripting perspective.
So remember, features like pop-up can be added to missiles (just wait until you guys get the full advanced torpedo mods

), but its important to remember two things: 1) Does it actually add anything to missile performance in game terms? 2) Can the AI use it without consistently causing the weapon to run errantly?
There is so much that can be done with every aspect of DW to be honest... too bad I'm not smart enough to start a for-profit company based around such an endevor.