If the cast iron tee is threaded rather than leaded, you can get a PVC or ABS adapter that will just screw into the tee. If it is a leaded joint, then the easiest way would be to use a banded coupling and leave the connection alone. A banded coupling is not a Fernco. It has a stainless steel band the entire length of the fitting. A Fernco is not approved for inside work that will not be buried. IMHO, although PVC and ABS are different materials, there is not practical difference.