Ary! It is important what kind of C language you are using. In example Raisonance syntax will be diffrent than Keil, I think, but I don't know Keil's C and I am not sure about this.
And this is important, which way your memory is connected to the micro. Is it CODE MEMORY or DATA MEMORY?

But I think you are using data memory. Isn't it? In Raisonance you can define external memory block the same way as in your example:

xdata at <address> <type> <name>

i.e.

typedef struct
{ ....
} MyTypedef

xdata at 0xFF MyTypedef Table1;

Good luck!
Jacek