I've done many crayfish surveys in the UK, never seen a markbled crayfish in our rivers, but thats not to say if it was released that it wouldnt be a problem. The "crayfish plague" is already existant in the UK white claw.
The problematic crayfish here is the american signal crayfish, thankfully it seems to be sticking to waterways in the south of england. If you get a signal crayfish in your river you can practically say good bye to the native species in that entire river system, this is not due to disease, it is due to non-native species out competing the native species. The signal was introduced into the UK back in the 60's when they farmed them for food.
Just a little on the UK situation!
My advice would be to contact you local authorities in Belgium and get advice on how to proceed from them.