Hi pittbull,
Yes you can do what you propose but since the XGATE memory map isn't paged, you can also define a larger segment using global memory.
SEGMENTS
....
RAM_XGATE = READ_WRITE 0x0FB000'G TO 0x0FCFFF'G ALIGN 2
....
PLACEMENT
....
XGATE_CODE INTO RAM_XGATE;
....
If you use different ways of defining segments just be careful that you don't define the same segment twice.