Hi,
developping custom drivers involves taking the current drivers as an example, and implementing the specific db access functions.
you can find an interesting thread there:
http://www.llblgen.com/tinyforum/Messages.aspx?ThreadID=5657
There's not yet an official documentation for driver implementation
Yet, in the download section, you'll find the sdk, which contains the source code for all the native drivers. There are not big projects so it should not get long to figure out what you need.
You also need to develop a dynamic query engine. You also have the source code for the native DQE in the sdk or in you RuntimeLibraries/SourceCode folder.
Current llblgen users are developping custom drivers which you can find at:
svn: //www.sd.nl/LLBLGenPro/CustomDrivers
With the source code for all those various drivers, you should be able to find out what you need to implement. If Progress does not have much specific syntax, then you should be fine starting from an existing driver and changing a minimal amount of things.
Cheers