You can use CrossStudio for AVR with a third party supplied AVR GCC toolchain. To do this you must start CrossStudio for AVR from the command line with the -gcc command line option.
The location of the AVR GCC toolchain is determined by the global macro AVRGCCDIR. To set this use the Project > Macros... dialog and specify the AVRGCCDIR value in the global macros editor.
Note that no CrossWorks libraries are usable when CrossStudio for AVR is used in this way.