If I understand correctly, you’d like to re-use the training set that was used to fine-tune the original NER model (i.e. the teacher), but now include the new annotations for the student?
To be honest, I don’t know how well the student will be able learn the new entities since this depends a lot on the number of samples, the relative weight of the knowledge distillation term in the loss etc.
PS I’m also not sure if knowledge distillation is the right approach here - I think your use case is an open research problem, so I’d suggest seeing what has been tackled in the literature before committing too many resources with my half-baked suggestion