Here's a specialized forum for CCS compiler: https://www.ccsinfo.com/forum
if you don't find the driver there, post a request, may be someone has written it already.
CCS comes with a driver for some other A/Ds from the AD77XX series. May be you can modify it for AD7799. May be you can find something close on this forum or on CCS fourm. Sear ch for AD77*.