Yes, you can bring in your own code using the j2 Developer tool. This includes own code, spreadsheets, equations, engine deck information, flight test data, wind tunnel data, low and high order CFD data, digital DATCOM. The j2 Universal Tool Kit is a defacto framework product providing a powerful intergation platform. j2 have not yet come across a code in use that cannot directly integrate into the j2 framework that can then add fidelity to the aircraft model.
The key strength of the j2 software is that it is able to use the legacy code that is trusted and proven in the first instance, show how the j2 software uses the existing code and then to demonstrate how this is enhanced using j2 in value terms.