The way I feel you are tyring to make your system is acquiring the video from the ATI board and using it with IMAQ Vision.
This feature is indeed not supported, but you may find solutions with the manufacturer of the ATI Video board. If they do have a driver (DLL) with some SDK or similar to acquire the video you will find it easy to interface with LabVIEW, if this is what you are using or other programming language, get the video in memory as a 2D array of numbers and then convert this array into the image data type for IMAQ Vision. Please check with the manufacturers if they do have this options, if not then I hope another enthusiast may have a solution to offer to you.
Good luck!
Nestor Sanchez
IMAQ/Motion Support
National Instruments
Nestor