Checkout this video how to do this:
The main difference from gerber and ODB++ is the missing components, attributes and net information. In original gerber files no layer type defined, but in later version this can be included. In Gerber X3 it is possible component information are available and in gerber X2 it is possible to contain attributes.
If you have no such information you have to do some tasks:
1. Define stackup information (e.g. set signal layers) in the matrix dialog, more details here
2. Add component from e.g. Pick and Place file via component csv import, more details here
3. Create net list, see more details here
4. For existing libraries you can get part information from your library, e.g. using the EPL
5. Set the board outline, maybe you have one gerber layer with the profile
With all this information you have a full ODB++ Design and can it export or make all analysis and create reports.