In general you should have a look at
AN2408 and
AN2287 which have detailed descriptions of the expanded bus and how to use it.
For your specific question the answer is very simple. You can access the SRAM using any opcode that loads or stores data to memory. The opcode must contain an address in the RAM.
For example, to read a value from a table in flash you might use the opcode: LDAA 0xD000.
To read a value from a table in SRAM you use the same opcode and change the memory location. If the SRAM is at address 0x4000 to 0x7FFF then you could read a value using LDAA 0x4010.
If you program in C then the only thing you need to do is place your variables in the correct memory range and the linker will take care of creating the correct opcode.
The most important consideration is how you map the SRAM into the memory map of the S12 and this subject is discussed in the application notes.