Hi,
you can check if the functions provided by ADL will work for you. You can get ADL from:
http://developer.amd.com/tools-and-sdks/graphics-development/graphics-development-sdks/display-libra...
Please have a look at the functions described in: Modules->Display Related API -> Timing Override
You will find functions that allow you to add new video timings by specifying resolution, refresh rate, sync width, sync start etc.
Chris