I wrote up a brief explanation of the builder (for learning rules, but it also applies to all other Nengo objects) here. There is also documentation on the Nengo build process here. You can probably write custom builder code to do the FPGA compilation step.
Another approach would be to use the standard Nengo build process (i.e., just create a Nengo simulator object), and iterate through the various Nengo objects (ensembles, connections, etc.) to extract out the parameters you need for the FPGA model (e.g., iterate through all of the ensembles to get the gain and bias values).