No, Beuchat foot-pockets are not compatible with Sporasubs - they have different rails and also the holes are not positioned identically. Sporasub does not make foot pockets with replaceable blades anymore, but Dessault does, and they are now even better than they were when Dessault made them for Sporasub.
You can get Sporasub foot pockets for example here (26€ for a single foot pocket):
Chausson seul palme ELITE Sporasub(la pièce) Sporasub Palmes pièces détachées
I am not sure if these are the old Sporasubs, or rather the new Dessaults, but I think the later
EDIT: since in the same store they offer the pair of Sporasub Elites for only 45€, you better buy the complete fins instead of a pair of footpockets (2 x 26 = 52€) if you are going to order in that store. However, you may be able to find a better offer elsewhere
