c6678 interface spartan6 through SGMII?

Electrically it's certainly possible.  The c6678 SGMII transmit and receive lines should be connected to the FPGA SGMII receive and transmit lines respectively through blocking capacitors as shown in the Keystone I Serdes Implementation user guide.   You will have to determine whether the impemenation in the FPGA is emulating an SGMII PHY connection or an SGMII MAC connection and configure the SGMII negotiation accordingly.  There is more detail on the possible connections in section 2.4 of the Keyston Architecture Gigabit Ethernet (GbE) Switch Subsystem user guide.