Don't know how on S12XE, but S12XD and S12XA XGATE can't access external memory and EEPROM. Only all onchip RAM, whole 2k registers block and 30kB of flash.
This is also true for S12XE devices. The XGATE's memory map does not contain any external address space. There are only two ways to let the XGATE access data from an external memory:
Let the CPU12 copy the data into shared RAM space.
Switch into single-chip mode and drive the external memory interface through GPIO. (...but, wouldn't really recommend that )