I have learned that Allegro will only import "Allegro" generated gerbers. Evidently, Allegro has a very specific format in the Gerber the tool generates and only that format is supported by their import tool. Any chance FAB3000 can be configured to generate the Allegro specific format?
After doing some investigation and a bit of experimenting I've been able to get the FAB3000 generated Gerbers to import into Allegro.
The changes I made were:
1. first line of the Allegro generated RS274-X is an * character with no other characters. I added this to the FAB3000 generated files.
2. no blank lines, an empty line always has a "%" character. I added the "%" to all blank lines in the FAB3000 generated files.
3. the line starting with "%LN" in the FAB3000 generated RS274-X files causes the Allegro import to error out. I removed the line.
4. in the examples of Allegro generated files I examined, the "%AD" definitions always preceded the "%IPPOS". I moved all "%IPPOS" line to follow the "%AD" lines in the FAB3000 generated files.
With these changes the files imported with no problems and look as expected. I'm not sure this is a complete list of the issues since the Gerbers I was tested are very simple, but at least it solves my immediate needs.