In order to do that, you need to handle the bio functions. These functions has to read or write into some buffers which are sent or updated with received data from BLE. Is not an easy task, but I managed to do it, with DTLS.
Think about how the sockets works in a OS. mbledtls expect data to be written and received from sockets. If you implemented the sockets over BLE, you’d be able to make it work.
For TLS (not DTLS) you need reliability. If you assure the data is received by endpoints and it’s received in order, TLS can be implemented as well. But, I’d go with DTLS as it handles retransmissions, packet order and so on. It’s easier to implement.