First of all, thank you everybody for your quick response, but I think I didn´t explain it in enough detail.
Actually we don´t use any external RAM. We just want to use the XRAM lines to connect the CAN-controller with the Core. To reach this goal, we built a wrapper around the core, so that we can direct the XRAM lines to the CAN-controller.

We write all our programs in C. So the KEIL-compiler should use the right instructions and convert our code correctly into assembly code?

To switch from the internal RAM to the external RAM we used a simple condition, to check if the value of the adressline is bigger than 0x8000. We realized that by implementing the instruction in the wrapper VHDL-file, but we didn´t change the core yet.
Maybe we have to switch to the external lines in the core and not in the wrapper?
Do we have to enable something, if we want to use external lines?