If you connect it in parallel with the dynamo it'll work. Though no switch.
If you connect it to the output of the supernova it may or may not work. If it does work you'll get standlight power from both the front light's capacitor and the one built-in to the rear light.
this questioned has been answered before, but hey, have an older supernova front light and about to pull the trigger on a B&M rear light. Will they work together? I'd rather not have to buy a supernova rear if possible.
This is the rear I'm looking at
https://www.sjscycles.co.uk/lighting/busch-muller-toplight-line-plus-rear-dynamo-light/
Thanks