I think Schema therapy is a type of CBT, so some of the therapists who call themselves CBT might actually be trained in Schema and could provide it. Maybe your backup T would know some practitioners? There is also a CBT group format for BPD that is called STEPPS. I don't know how common it is since DBT seems so popular these days.

Would you be able to continue with the DBT group indefinitely while seeing the backup T, or would the DBT people not allow you to keep going without an individual DBT therapist? It seems like one of the roles of the individual DBT therapist is to be available always so the client can call for support in using skills to regulate their emotions, not self-harm, etc. If you don't need a T for this purpose, i.e. you don't need to make calls to the T outside of your sessions, then maybe not having an individual DBT counselor would be OK with the group folks.

Good luck finding what you need.