While (as long is it worked reasonably with the UI, and wasn't a micro nightmare) I'd love it if this was a thing, I doubt it will be practical to include things like this in the game, because if it's anything like HoI1, 2 or 3, ships can stay out at sea indefinitely anyway, only returning when they need repair (they've even been upgraded to the latest AA and the like at sea in past HoIs - Naval Underway Refitting FTW! - and I can see why this is, as having to return each flotilla (and now individual ship) back to port to get radar or new AA would be fiddly).
From what we know of the mechanics so far, we could potentially shorten the ranges of u-boats, and then (if it was possible) mod in a unit that acted as a floating source of supply, which then extended u-boat ranges, but I'm not sure whether the range shenanigans required to make this feasible wouldn't mess up other elements of submarine warfare, and same for a similar approach to a naval supply ship. Just thinking out loud here, not really sold on the idea, but maybe it'll lead to something better?