It's possible now, but as you say, a lot of work. I have some hope that the add-on will add a dialog in front of the normal campaign choices allowing you to pick which country you wish to play for first.
Making the subs, etc is certainly possible
As Peto suggests the campaign will take some work. Personally, I ignored the allied forces pretty much since the surface (or CV) engagements are pretty horrid, and impossible to properly script so that the right ships sink (what's the point of setting up the battle of Savo if all the IJN ships get sunk, or even if it's not close to the real results?)
So for an IJN sub campaign, all this would need looking at.
tater