As it is, it cannot.
I agree a single capital ship designing tool could allow to create BCs, BBs and SHBBs. However such a tool would need to allow the player to set different values. For example, choosing the displacement, which would unlock superior levels of armour and firepower while increasing the industrial cost and reducing speed if set high, or limit firepower and armor while decreasing the cost and increasing the speed if set low. Such a system could allow you to design a Yamato, a Richelieu or a Scharnhorst, but would be quite horrible to balance.
The variant system in HoI4 doesn't do that. It gives you a base value that represents the bare minimum of what a 1922 BC or a 1944 BB can be, and from there you can increase Firepower, Speed, Armor and Reliability. You can't reduce one stat to improve another, you can only go upwards from the base level. It's easier to balance, but it means BCs, BBs and SHBBs are different units.
The devs went for the second option : now it's too late to envision another system, the only good out come would be for this second option to result in an accurate tech tree.