The ADuM2400 is strictly a digital device, but I2C is only digital as well. It is just that I2C is open collector and bidirecional. There are buffers that allow conversion from 2 bidirectional lines to 4 single direction lines.
There is nothing stopping you from cascading several I2C isolators, They just need to be connected Side 1 to side 2 or side 2 to side 2, Connecting a side 1 to a side 1 will not work. There is even a new isoPower version of the ADuM1250 available ADM3260 it is rated at 2500V basic insulation, but it has the creepage you need, and you can likely cascade a couple of them. from each direction.
If you need the full 5kV then you can cascade ADuM2250 and use ADuM6000 as the power supply.
Best Regards,
MSCantrell