The only protocol supported by both AB and GE Fanuc is Modbus. For the 90-70, you ned a IC697CMM711 communications coprocessor module. It has two ports, either of which can be configured as a Modbus slave.
That means that your SLC would have to have the capability to be a Modbus master. I don't know enough about the details of the various SLC models to be able to tell you if that's possible. Maybe Ken Roach can answer that one, or else get in touch with your local distributor.
Another possibility has been suggested on the GE Fanuc forum. I haven't done this myself, so I can't personally vouch for it, and I have not been able to find an application note on it.GE Fanuc offers a small HMI unit called the Datapanel 160. It has two independent ports. One port can be set up to communicate with the 90-70, the other to talk to the SLC. I think they do both DF1 and DH485. When you set up your tagnames table in this HMI, you can assign each tag to read from one address and write to an different one, and the addresses don't have to be on the same port.
The Datapanel 160 costs in the $800 - $1000 (US) range. You might try to borrow one from your GE Fanuc distributor to test the concept.